Are you one of "if it deals 400 damage I will go with 30 armor"? All those 300-400 damage attacks are easy to dodge (or at least you supposed to know how to dodge them), usual target damage for defense is 140-180, if it's lower you don't really care, if higher you better learn that specific attack to avoid it. Even if you go beetle armor without any warding, and having only shield (ankh for example) and celestial stone from defensive accs, you'll have 61 + 4 (shield) + 4 (stone) + 4 (aegis fruit) = 73 defense +10 defense and 10% DR from buffs, with same things on summonner you wil have 31 armor less (from armor difference). So vs 140 damage it's 51 and 79 hp lost, vs 180 - 87 and 115, so it's 35% and 25% less damage taken, pretty significant boost in survivability, I would say.Defense difference between classes does not make a large difference in survivability until you go full Warding on a Melee armor
After seeing your video it seems like you are the only one who talked about DPS and EoL speedkilling for some reason. No one (not me for sure) ever said that terrablade is must have for killing bosses in 30 seconds, so don't know why you even bring that topic answering me. Game is a bit more complex than killing boss with 30 armor so even moss hornets will be able to kill you from 2-3 hits if you will use that build outside the battle. In real playthrough there is no reason to even make such builds, you will spend much more time on farming/fishing than will save on boss battles (even with multiple tries).In general I'm not sure why you're focusing on armor and defense when we're talking about weapons and DPS
With normal builds that includes defensive and mobility accs and 2-3 damage accs terrablade perfoms the same as weapons from fishron/eol/betsy. Closest weapon by mechanics (piercing + directed ranged attack) to terrablade here is eventide (with wooden arrows) and flying dragon. They both have less piercing but more single target damage, flying dragon have less convenient attack but also goes through walls, and eventide obviously have no melee attack but its ranged attack have more effective range. So they all have some differences but in total they about at the same level. So that's why I suggest to actually put terrablade into that tier instead of having it available at same stage as psycho knife and the eye of cthulhu. It's not like it even will be hardlocked if we choose EoL variant, just kill EoL and you'll have your terrablade before golem as usual.