Some rough thoughts that could probably be organized a little better, but are at least hitting the notes I'd like:
As soon as I saw Themis's Pandae speech, I had a feeling that this conversation might start anew (on account of the whole "no regrets" angle).
My big picture:
By SE's own admission the Ascians were originally just supposed to be "doing bad stuff in the background" until they figured out what to do with them, so it was probably never a safe bet to root for them being vindicated as wholesale doing the right thing while the protagonists, the people who are supposed to feel heroic for playing the game, were revealed to have been making bad decisions and supporting the wrong ends for multiple successive expansions. That said, we were supposed to question it, of course, and they provided us many opportunities to do that.
Based on that, I'm sympathetic that some latched onto those in combination - "I like the antagonists, we're supposed to question ourselves" - and really believed in a narrative where that would turn things completely upside down to favor their favorite characters. And I'm sympathetic to the disappointment there. However, the are some issues where we're just beating a dead horse back and forth over whether people did or did not get what they expected/wanted, especially whereby some players act like having a different perspective is the same thing as the story itself being inconsistent or wrong. (Which is not to say it never is, either.)
And for the record, I loved these characters (at least, the complex, sympathetic people they are in retrospect), I just didn't expect a reversal of the narrative arc coding in the final act. I didn't expect them to win or be seen as having made the right call. I just liked them for the complex, well-meaning, flawed people they started as.
We saw the people of Amaurot desperately try to save their world. We saw the way Themis was warped by becoming Zodiark, by the prayers and desires of those who manifested him, by being re-manifest through that primal deity. We saw the way it inadvertently and tragically led to the Emissary, who was supposed to arbitrate and find the best path forward, uncompromisingly pursuing one flawed plan from one flawed perspective. We saw how the Convocation were inadvertently tempered to the same page. We saw broken Elidibus callously disown Emet-Selch and call him unworthy of being an Original for the implications behind "remember us", and yet the post-battle "restored self" Elidibus embrace this. As of Pandaemonium, Emet-Selch, Elidibus, and Lahabrea have ALL recognized, in that order, and in different ways, the flaws in the plan and execution and acknowledged that it was doomed from the start. Emet-Selch's final monologue in Endwalker, especially.
We saw Venat tell the Warrior of Light she wouldn't take their story for granted and would try to change the fate of the ancients, only to fail. We saw her agonize over who to bring into the fold, terrified that alienating Hermes from the convocation or allowing him to find out that his own "unbiased test" had become compromised would lead to the entire society finding out the results of Meteion's survey and not only undermine her mission to save them, but create conditions that would destroy even the Warrior of Light's one chance to defeat Meteion (which was itself set in motion by her future self) in the future.
Unfortunately we skip right from there to post-failure. We skip from there to 75% of the ancients being dead, the Convocation being tempered, and the society forsaking its way of life to sacrifice more and more to the primal to deny sorrow and restore paradise. We skip to it being too late, and we only see even that through Venat's metaphorical walk down memory lane, and that gap lets people deny the rest of this and make assumptions that better fit with what they wanted. (And yes some make assumptions here to support hating on the ancients more than the story backs up as well.)
But, in my humble opinion, that doesn't mean it's inconsistent for Themis to say he's glad to see life goes on and that he is unburdened by regret. Emet-Selch also gave Venat credit for her foresight, but says he wouldn't change his actions, too. He played his part and was true to himself, as was Elidibus, and Lahabrea recognizes his arrogance and hypocrisy as just that - and now we understand why he was broken in some ways even before the Final Days. Venat doesn't get fully vindicated, either, they dedicated a whole cutscene to her recognizing that, "I breathe fire and torment. I birth a world of suffering to mire and plague."
They all did the best they could to give us - however imperfect by their standards - a future. They all made difficult decisions that had good and bad outcomes along the way. It's understandable that some people identified more with the more-immortal, more-beautiful, more-perfect-society ancients than the short-lived, squabbling sundered, who stand in for the players - imperfect people with imperfect lives full of struggle and sorrow in equal measure to their success and joy. It's understandable that when the story let them fill in the gaps with interpretations and perspectives, they chose ones that fit that identification and desire. It's understandable the story didn't go that way, and understandable they're disappointed.
And I understand 6.4 Themis is going to bring up some of that old disappointment for some people, but is it that new or noteworthy or inconsistent with the transition from the broken Elidibus who told us there was no common ground to ever be found between himself and the sundered and disowned Emet in 5.1 to 5.3 vs. the restored Elidibus who understood Emet-Selch and recognized his hopeful naivete and the ways it went sideways at the end of that arc?