Genosha is, in itself, not a bad thing.
We knew Fassbender was in this, we know the studio/Kinberg likes the Magneto character, we knew he would have a substantial role and a sub-plot.
For him to go off and establish Genosha is a logical step from the end of X:A. I think it's more to do with what they do with it.
Genosha could have been a set-up for a House of M spin-off, or something like that.
It sounds, from rumours, like Dark Phoenix might destroy it (an equivalent to the D'Bari world in the comics and also to Cassandra Nova getting the Sentinels to destroy Genosha). I'm not fond of the choice of seeing it destroyed, though I haven't seen it in the context of the movie of course.
There's no way Jean can redeem herself from destroying an entire island of mutants. Surely she can't remain alive at the end of the film?