It Hydaelyn sundered herself, why doesn't First Hydaelyn handle the flood of light?

[Lyth]

...

When Ardbert's team came to the Source, they had been deceived by Elidibus into believing that the only way to save their world was to destroy ours. Minfilia's decision to give up her friends and past life to save the First was what set the Warriors of Darkness on the right path again. It had to be a member of the Scions, as their Source equivalents. You could argue that had anyone else intervened in Minfilia's stead, then Ardbert wouldn't have had the growth that he needed in order to save us in turn during Shadowbringers.

The Hydaelyn question depends on what sort of impact Her absence will have on both the lifestream and the stability of the remaining shards in the future. If things start to destabilize over time in Her absence, that may explain a limitation around Her involvement.