Hickman clearly wanted to set up the general world the X-Men/mutants are living in before doing anything like that.
With that said mutants have been powered without the Phoenix. Apocalypse himself repowered Polaris and Sunfire. High Evolutionary repowered Magneto. They should be able to figure out how to repower the depowered mutants.
Of course could Krakoa hold 16 million mutants or where else would they put them? Colonies off the planet?