Okay, let's go in order of appearance.
First of all, the main reason this weapon is so powerful because it's post-Moonlord. What did you expect from a sword that requires the Moonlord to die multiple times, plus requiring many different swords from different stages of the game? At that point there's not really any "higher threat" that would make the Zenith too overpowered in its stage.
It's not powerful, it's OP. Not a single major update i could think of added an endgame item with click-to-win functionality. Moon Lord drops before were powerful, but strongest of them required continuous use of mana and had no homing ability. Plus, requirement of many swords you've mentioned is a complete bogus — you get all of them through natural progression. The only serious randomisation here comes from Enchanted Sword. Afaik it became much easier to get in 1.4, but i haven't checked it yet.
Second, the Zenith got its name from the same word, which means "the highest point reached by a celestial or other object" (according to Google).
In short, the Zenith is the most powerful sword in the base game, so it being the "highest point (AKA highest tier sword)" would fit the weapon knowing its potential.
Changing it is most likely a bad idea.
It's highest point, and this only supports my idea of making it Master exclusive. As for the name, asking Terraria naming and graphic design convention to have any meaning is a bit too extreme. You beat the game, so you get Zenith, Last Prism, Celebration and.... this Space Dolphin boi. Enjoy — at least now you can craft this annoying Cat Sword away. If, however, you demand for the name to be significant, then google again. Zenith is swiss watches brand, so, naturally, reintroduced "true" Zenith should be called Omega. I'm not even joking — this word also has significance in this particular in-game situation, with sword in question being literally the last thing player gets.
Third, every current Master mode drop is either a pet or a mount. Besides the Toy Tank, none of these cosmetic drops deal any damage. So why would the Zenith, a melee weapon, break such a pattern? Sure, Master Mode is tough (despite there being only stat changes for the bosses), but the exclusive items are for cosmetic use only. Plus this would make obtaining the Zenith easier then how its currently obtained.
Heeeeeelll No. I certainly wouldn't call Master mode mounts to be mere cosmetics, especially considering that devs now have plans to nerf Black Spot — it turned to be so powerful it made wings obsolete. Why would Zenith break a "pattern" then? Because the weapon that breaks any remnants of balance right after you completed the game can also be considered glorified cosmetics. The only thing you would use it for, aside from world jumping, is to farm all the other collectibles you've missed. Either this, or just hang it in your trophies room as a reminder. Or go to PvP multiplayer and break all competition there, but i'm not qualified to talk on this particular topic. Sure it can be banned there, right?
Here goes important part:
Why would i then introduce you to the possibility it should be Master exclusive? Because it broke intricate balance of ML drops within the context of different World modes by introducing an item one tier higher than anything else, which now leads to these ridiculous demands to make Zenith Gun, Zenith Spell and Zenith Staff. No, they don't care about Last Prism, Celebration mk2 and Terraprisma, as they aren't as OP. And you can't realistically argue against that — people from non-melee community want to be represented, because they clearly understand what Zenith implies. You mentioned "patterns", so how about this one: you have to beat the game on at least Expert to get fastest Wings and accessory with infinite flight functionality. Yet you get click-to-win sword for completing the game on Classic.
This situation leads to two possible and popular proposals:
— If you recognize Zenith as legit and balanced ML-tier weapon, then it should be nerfed a bit
— If you see it as Bragging Rights Reward one tier above, then other classes should get their Zeniths
Both are flawed, as Zenith crafting tree implies its above-all role, and, on the other hand, swords are arguably the only type of weapon in Terraria to have such a significant cultural/mythological footprint IRL. So i invented the third one:
— Zenith should be nerfed a lot and reintroduced in its current form as Master item
In fact maybe not even "a lot" — in vein of first proposal Arkhalis-alike with bigger (but not screen wide) attack arc and single target DPS well below 20000 also works.
Plus this would make obtaining the Zenith easier then how its currently obtained.
How so? Am i missing something obvious? Again, my idea was for ML to (always) drop some thing on Master, that will be used to craft "true" Zenith.
Fourth, the only reason the Terraprisma is hard to obtain is because the boss that it drops from oneshots you at the time the boss has to be fought to obtain the summon weapon. It may be weaker then the Zenith, sure, but it's proven quite powerful and stands as a good alternative to summon weapons obtained after it (even supposedly being more powerful then the Stardust Dragon with the right setup). Also, you only have to kill the boss once at day to obtain the Terraprisma compared to killing the Moonlord at least twice just to get two ingredients for the Zenith. You also need twelve more swords, five of them combining into one, and a third of those twelve swords are obtained after the mech bosses.
Have you actually tried crafting the Zenith, or are you just saying it's easier to obtain just 'cause?
Killing ML twice or more, especially with the usage of sequential ML drops is much easier than killing EoL during the day post-Plantera. Because ML doesn't oneshot you, you guessed it. I did the first thing enough times in my 1.3 world, but simply can't do the latter w/o SDMG and Vortex Wings. I dunno, maybe minecart cheese hype is real, but i hate using minecarts for things other than travel and sick stunts. Twelve more swords again are not the issue, because by the point you have two ML swords, you'll get most of them anyway. You can, furthermore, even skip entire sword progression throughout your walkthrough, but then you probably shouldn't insist that getting them with post-LC/ML gear is a legit challenge or even a nuisance. Again, Echanted sword is an exception from my experience (the last thing i need to craft Zenith with 1.3 character) and i would hate it IF it still is a legit bottleneck in 1.4 worlds.
Deal with it, because I doubt it will ever be changed in your favor.
You added this line because you secretly suspect i might be right, is that correct?