Yeah... But Hydaelyn is massive... Like... How do we know what is even going on at that scale? Alexander is large enough to sustain life, apparently... So, hypothetically... What if a group of Goblins, inside Alexander, decided to worship and summon, IDK, Enkidu... Where would Enkidu drain aether from? Hydaelyn or Alexander? He would be summoned inside Alexander, the same way current Primals are summoned on Hydaelyn... I'd imagine he'd drain aether from Alexander, and Alexander might take measures to combat that...
So... Where does Hydaelyn drain aether from? Space? *Looks up in the sky* Oh right, we have Zodiark embodied as the moon, as a dead lifeless husk... Almost as if someone drained all his aether...
As for who summoned Hydaelyn... No idea, not much room to speculate on that, it's a "If God created everything, who created God?" kinda thing... I'm not even sure how many Hydaelyns there are anymore... What with all this talk of alternate realities starting to surface, but even before that we had an oddity with the Blessing of Light... There would seem to be multiple Hydaelyns... Maybe Hydaelyn summoned Hydaelyn... Maybe C-137 Hydaelyn died, and all the people living on it prayed for Hydaelyn to save them, so they summoned C-138 Hydaelyn...