I never played FFTA but did their decision in that game also include killing billions of real people and later sacrifcing their souls to a false god? I honestly read a summary and it sounds nothing like FF14. The biggest difference: FFTA was playing in a dream world, while FF14 is in the real world. Of course its easier to destroy something which is truly not real which in turn seemingly also saved those that got pulled into it. In FF14 the Ascians simply cant accept that their time is over and that the world has gone on.
The funny thing is: We once were an Ancient too and probably the 14th member. And yet even though we lived in that world and were part of the leaders of Amaurotine we still did not want to be part of the Zodiark plan. And other Ancient ones also were against the future plans of the Ascians..so much that they call this the first true conflict that their race had and that they called Hydaelyn to go against them.
So sorry "we" once were looking at it from their side and "we" even lived through that and still decided to go against them. Maybe because killing who knows how many new lifes to maybe get old life back (who sacrificed themselves on their own free will) is just bad? At that time they simply could have stopped and those that survived it all could have rebuilt their homes. The planet would be whole and the souls not shattered. But no the Ascians needed more. Could not accept the sacrifice either thanks to their tempering or maybe because of their inability to go on. And thus in the end the planet was split and their race gone.
Its not even the darkness metaphor. Its them being people that are laughing at destruction and death they have created. Building whole empires for the sole reason of causing another rejoining and thus genocide. Its Emet saying how he sees us as not alive (yet makes children with us..)..its not just a metaphor, its their whole actions. If they showed regret in doing it...but no Lahabrea loves it. He laughs when Bahamut is free to slaughter the world.
Its also kinda funny since Yoshida stated in an interview after the release of the expansion that people should not forget that the Ascians and Emet killed millions of people. He also pointed out that some are way too fast to fully believe Emets side of the story and thus see Hydealyn as bad and that we should wait until we get her view on it. I could be wrong but that does sound like we will get more reasons on why this is the right and good thing.