Even if Miggy didn't do what he did, I figured that Hydaelyn would lose her power entirely eventually. We've basically been grossly abusing her power this entire time to slay Primals and other "impossible" foes, and the Ascians directly mentioned that "each victory came at great cost to her". It could have been possible for us to literally reduce her power to zero, which might outright kill her.

Miggy stopped us from continuing to sap her power further. It may be possible that he's actually saved her, and perhaps even taken her place as a source of being chosen via his ever watchful eye.

Besides, we kind of proved that we don't need the Blessing of Light to kick an Ascian's bum