Zodiark was unfathomably huge as far as aether goes. Trying to do my own math here, if we just ballpark that the Ancient world and its denizens have twice as much aether in them as the current Source (I hoped the EW fisher stoyrline would shed light on this, but it doesn't), then it stands to reason that making a Zodiark now would require sacrificing the entire population, or their 'ambient aether' equivalent. That's a lot of aether, and it logically tracks with his fight; we only fought him at half-ish strength, with a pilot that had barely any experience, and he's still among the trials in this game.
For 'ambient aether drain wouldn't have been as big a deal pre-sundering' to hold, the primals threatening it need to be of comparable power to Zodiark. And they're fractions of his full power; Ramuh was made with a stockpile of crystals, Alexander is powered by what's basically an aetheric vacuum cleaner. Eureka we don't know the origins of, but we do know it wasn't making things nearly as fast or as frequently as Zodiark. Hell, while he's not directly comparable, Eden was just one sundered Ascian jacked up with a bunch of light aether, and it still nearly scoured a whole shard clean.
Yeah, I genuinely think that if Alexander walking was a potential Calamity's worth of aether drain, Zodiark doing something as significant as putting out an eternal barrier probably would've been much worse.
And my comments to Turk are just underlining the strangeness I find that the people allegedly defending the Ancients are the ones insulting them. The thing about the Ancients falling to what they did is that it lets them be as great as they were: if they were only felled by an essentially unstoppable force, then it's not a failure, it's just a loss. Similarly, Hydaelyn being the result of an Ancient group getting mass support, as the EE3 tells us, means that the Ancients went out on their terms. Even if they could never have been perfect, the Ancients were all they could have been; they were their best and did their best, and the story never takes that from them.
...but the argument of 'they could have solved this' does take that from them. It takes away their scientific intelligence by saying that they weren't smart enough to come up with your solution; it takes away their social intelligence by saying that they could easily have been swayed by a lying demagogue. It tears apart their political integrity by saying that said demagogue could've gotten that power in the first place, even though we see Lahabrea recognize and act on that exact problem with Athena.
By saying that there was some obvious perfect solution that they just didn't take, we infantilize them. That their enlightened masses were easily fooled, that their greatest minds couldn't see the obvious. That they were merely naive babes who were hopeless without our help, instead of mature adults who couldn't stop the inevitable, and eventually stopped themselves going further.
I think the Ancients deserve better than that. I can't stop you forcing them in the kiddie pool, but I can do my part by making this the last thing I say on it.