What's the point with that quite lackluster story in Endwalker?

[Lauront]

Oh no, people aren't trying justify Emet's actions at all.

They mostly dislike how everyone in the story seems to ignore the implications that Venat/Hydaelyn was directly/indirectly responsible for a lot more harm then just the Sundering yet is painted in a far more positive light then her peers simply because her path was framed as being the only available option for the world's survival by necessity due to how all the time travel/causal loop nonsense functions.

It's that, but also the fact that they've framed it as a function of her belief (and to add to this, wrote it in such a way as you can sustain an interpretation like Brinne's, that she romanticised the sundered world based on the WoL's account to some degree), while also leaving some open plot threads that may have been utilised to lead to alternative outcomes, leading the 'necessity' part to be brought into question as well, as well as leaving the question: it worked, but at what cost? Understandably, for a storyline meant to close off the current arc and lead to a new one, many feel this was very messy, didn't do the plot points SHB had developed justice and left several other questions unanswered.

A common take has been that it's perfectly justified for the Sundered to do everything possible in order to survive. On the other hand, the same can be said of the Unsundered - it's a reasonable expectation for the Ascians to do whatever is needed to ensure that their people aren't wiped out.

Shadowbringers took that approach and presented it all as an unfortunate but necessary clash of like wills. If Emet won, his people would be restored and the Sundered would be no more. On the other hand, with the Warrior of Light's victory it was the Ancients who were eliminated in their entirety. The problem, then is that it is Venat who put both sides in such a horrific position in the first place but all sorts of contrived plot devices are pulled out in order to try and awkwardly justify it instead of just straight up admitting that it was pretty abhorrent and selfish of her to do as much.

Worse yet, the Sundered didn't even pass her 'test' because at various points they required the aid of not only the Unsundered but various other powerful entities such as the dragons and Omega in order to succeed. The victory against Meteion wasn't entirely their own doing, thus 'Dynamis' comes across as little more than a sad excuse to try and pretend as if the Ancients 'deserved' to be wiped out because they couldn't wield it effectively. (Which has unpleasant implications - especially when Garleans are unable to wield Aether as well as the other races and led to their persecution and near extinction...)

^This. The sundered as a whole are even more susceptible to it, with their souls being consumed, and even the strongest amongst them (outside your posse) succumbing to despair in several instances - and this is without the incident spreading anywhere near as far as the original did, or devastating the star to the same degree. We have to remember, enough ancients survived the entire ordeal to summon Zodiark in the first place, so it's not as though they were wilting lilies. The problem is that their creation magicks were also being hijacked in the process (the unaddressed subterranean sound, which the Watcher mentions as well), leading to powerful, corrupted monstrosities roaming the star, while the star itself was dying off, leaving them as the only source of aether to fuel into Zodiark - a being which itself protected the sundered world for 12k years.