I'm not going to call this ending a perfect masterpiece or anything but as a counterpoint to some comments, one thing I'm pretty satisfied with throughout this series and has stayed true through the end is how appropriately "Not Human" the Idaten act mentally. In the vast majority of series gods (or actual aliens) basically feel and act like somewhat different looking very strong humans, sometimes maybe with some quirks but nothing particularly surprising even if a human just lived a long time. The Idaten though feel more legitimately "off" and in turn true to their origins as the amalgation of thought patterns and such, something that has human external form as a copy but very much isn't human. Not giving a shit about things that would be deeply concerning to normal people and taking an incredibly relaxed view about means/methods so long as their basic goals (in this case the continuation of "humanity" or "train more get ever stronger" or whatever) are met fits. Miku too is unpleasantly yet unusually genuinely demonically intelligent evil, and her solution of basically finding a way to coexist regardless of means because actual victory is perfectly impossible, Idaten are basically forces of nature, works.
As readers who are ourselves very much human that doesn't necessarily make for the most grokable and satisfying outcome on one dimension, but on another it's kinda cool.