By successfully killing Moira MacTaggert before her powers manifested, the desperate remaining mutants in the current timeline would negate all the wonders, and triumphs, of mutantkind's Krakoan ...

Essex hijacked Moira Mactaggert’s mutant gift (dying and being reborn over and over, with full memory of the lifetimes she lived) to accumulate the shared knowledge of his clones’ lives ...