Lets take offensive builds scenarios, for instance:
1. If you fight Skeletron Prime with "500 DPS", and it takes you five minutes to beat him (after dodging/missing/etc is accounted for), then that's that. Five minutes is a substantial amount of time to continue to have to fight him, with the inherent risks that come with a boss fight.
2. Now, if you equip so much gear that you double your DPS? The boss fight is now 2.5 minutes long instead of 5. This is a huge improvement, sure, but its still substantial. That is still a non-negligible amount of time fighting Skeletron Prime, with all the risks inherent to that.
3. Now, lets say you go all the way and quintuple your DPS. The fight is STILL 1 full minute long. And so, while it grows ever shorter, the fight itself is still offering a proportional threat/time investment. Until you absolutely wipe the floor with them in seconds (which usually comes far later in the game, after multiple boss tiers), increasing your DPS is always a modifying factor on the boss fight, but it does not completely eliminate them as a threat.
Now lets look at defensive builds.
1. You fight Skeletron Prime (or any other boss, it doesn't really matter), and you have 50 Defense. And for the sake of argument, lets say that his attacks do 50 damage. This is normal mode, so your defense cuts that damage by half. Okay, cool, he's doing 25 damage to you per hit. That's a substantial chunk of damage still.
2. Now lets say you've finagled up a build with 80 Defense. His 50 damage attacks are now dealing about 10 damage per hit. That's pretty low, but its not nothing, so, okay, we're still in the clear here. You can't utterly face tank him, you might still die.
3. From here on, its all downhill. You keep stacking defensive benefits, and once you hit 100 Defense, his damage is completely nullified (down to a minimum of 1 damage). Even minor inclusion of Healing perks/furniture, plus the occasional potion, is going to be enough to stave off death from 1-damage hits. Once you've gotten enemy/boss damage down to 1, you've won. The enemy is never going to beat you, you've effectively negated their offensive power so far that they are no longer a threat at all (except if they have some sort of inherent time limit, like Skeletron Prime's dawn enrage, which is not the norm).
And so, therein lies the issue with empowering defensive builds. Offensive builds can merely reduce the duration of a fight, applying a sort of "modifier" that reduces, but does not entirely eliminate, a boss's threat. Defensive builds, as a result of the fundamental design of the Defense stat in Terraria, which cannot be changed at this point, have the potential ability to COMPLETELY nullify a boss's threat value to the minimum damage of 1.
And don't get me wrong: this is already possible. Throughout the game, via strategic use of various buffs and accessory combos, there are already a few bosses that can be tanked down to 1 damage, or close to it. Its not a good thing. I'm VERY hesitant to boost defensive combinations, because there are so many permutations and the risk of introducing "tank the boss down to 1" scenarios is a big concern of mine.
Just using a 5-accessory loadout in Normal mode, you can already get a 33 Defense bonus (one of them being Frozen Shield, which gives a Defense reduction bonus AS WELL). Make them all Warding, and then you've got a 53 Defense bonus. And I added 3 Defense to the Hero Shield and +1 each for Flesh Knuckle/Berserker Gloves, so THAT would bring the total to 58 in 1.4.1.
We've got Iron Skin Potion (8), Well Fed Tier 3 (4), Bast Statue (5), and if you are really careful, Dryad's Blessing for another 8, so that's 25 Defense from buffs, bringing our Accessory/Buff Defense combo up to 83 Defense. That's BEFORE armor! And if anything, that makes it worse, because the more we buff Defensive accessories, the more non-defensive classes (Summoners, Mages) can Defense stack until its abusable. Using the above combination, a Stardust Armor wearer could now reach 121 Defense (60 Damage Negation, plus the Frozen Shield bonus). You might argue that Stardust is post-ML, so fine, we'll do Spooky instead, for 30 Defense (113 total).
So I mean, what exactly can I do here? Every single point of extra defense can be accumulated up with all of the others, and once that "1-damage" threshold is reached, its over, bosses cease to exist as any sort of tier-appropriate thread. Someone mentioned making Warding give 6 Defense instead of 4? "Tack on 10 more Defense to the grand total". Make Warding give a % reduction instead of raw damage protection? This is actually a nerf to early game defense, as "-10 damage from a 30 damage hit" is far more potent than "-20% damage from a 30 damage hit". Furthermore, Defense reduction stacking is an equally terrifying beast, with Worm Scarf/Frozen Shield an already powerful combination that works for all classes. When combined with a Paladin Shield-wearer in MP, there are already some substantial "1-damage-boss" scenarios possible.
You mentioned increasing Expert/Master defensive scaling from 75%/100% respectively to 100%/200% respectively. This would actually worsen the issue!
Think about it:
1. Normal Mode: A player gets 50% Defense efficacy, against "100% damage" efficacy from enemies. This is what the basic game balance is designed around.
2. Expert Mode: Enemy damage is doubled to 200% efficacy (a 100% increase), while Player Defense is increased to 75% (a 50% increase). This works, because Expert is MEANT to be harder than Normal mode. If we doubled the player Defense efficacy to 100%, then the scaling is completely even . . . all you are doing is doubling the damage taken and doubling the damage reduced. This doesn't even maintain the status quo, because now the player has a 6th accessory slot to use, so the previous Defensive loadout actually has the addition of Worm Scarf/Celestial Shell, for MORE defense (not to mention another Warding prefix). That 58 Defense from before? We can call that 65/69 (Day/Night) with Warding Celestial Shell, or we could go 62 Defense + 17% Damage Reduction with Worm Scarf. If we maintained the defensive ratio, then this would mean it was actually EASIER to nullify enemy damage in Expert mode.
3. Master Mode: Enemy damage is tripled to 300% efficacy (a 200% increase from Normal) and Player Defense is increased to 100% (a 100% increase from Normal). You propose increasing Player Defense all the way to 200% efficacy . . . this would be way out of proportion. This would actually be a 300% increase in player Defense efficacy, from 50%->200%, which actually leaves enemy damage trailing behind, and it would be even easier than Normal mode to "1-damage" bosses. And of course, we are now talking about 7 Accessory slots, so we are up to 69/73 Accessory Defense + Worm Scarf. A Stardust Armor user would have 107 Defense, and with the proposed 200% defense efficacy, they'd actually be nullifying 214 damage. That's enough to reduce most of Master Moon Lord's attacks down to 1. In Stardust Armor.
So, the ultimate problem here is that boosts to defensive systems are an extremely delicate situation. Every single defense point added is combined with an ever growing tally of assembled defensive boosts, with an ever looming "critical mass threshold" where bosses become non-threats due to dealing 1 damage. Furthermore, the actual amount of defense available to ALL classes is relatively huge, which means that too many missteps here will result in Rangers, or even Mages and Summoners, being able to facetank enemies with the right loadouts. There are combinations where this is already possible, with certain classes/bosses. I dare not make that problem worse.
This is not a system that can be reworked. Not now. I've made some pretty significant changes, but I'm in no position to tear down the entire structure of Terraria's defensive systems and rework them from scratch. Not only do I not doubt it would be denied outright, I'm simply in no position to attempt to account for 20 bosses across 3 difficulties and 4 different classes. The sheer amount of rebalancing required would be phenomenal . . . I'd have to review/adjust every single piece of armor and every single defensive accessory in the game. Hundreds of items, dozens of bosses, multiple difficulties . . . the scale of that is tremendous, and I wouldn't really have much of an opportunity for second chances. Realistically, a better system from the start might have been a Damage Reduction % system with decreasing benefits, so that even the mightiest of tanks couldn't nullify damage, but that ship sailed in 2011. I can't bring it back now.
The best I've been able to do was give some % damage reduction bonuses to some of the Melee armors in 1.4, and a handful of minor defensive accessory boosts for 1.4.1, which have already been elaborated on. I honestly haven't had the time to thoroughly investigate the already-existing Defense stacking implications, which I know exist. The echoing repercussions of massive buffs to this system would take too long to evaluate, and I'd probably have to make corresponding counter-nerfs to other items or bosses to account for it, which really negates the point of such buffs in the first place.