it would seem everyone needs a refresher on why they did time travel in the first place. Eight umbral calmaity hits black rose is used Garlemald magiteck is non functional and other city states are void of aether think the empty but on a larger no honestly think Novrant and the first cause its basically that but eorzea. Emet-set and elibus ( unless they were killed though out the timelines)still exist in that doomed era. I don't even know if middy can reverse that emptiness, not even cid could.

What the exarch said was true the tower needs to go back to eorzea to ensure the 8th calamity of that nature doesn't happen. Until it leaves the first, there will be a calamity of epic proportion that will necessitate the use of the tower again. Maybe when we defeat zenos the tower will vanish. But until it has nothing will have changed.

I will not discuss the 3 timelines of zelda here. My attempted knowledge comes from Chrono Trigger and loz MM ( song of time anyone)

It is simple cause and effect.