Making Wanda & Pietro non-mutant is one thing, but in retconning Mags' parentage you basically nullified several interesting relationships with untapped potential that we'll never get to see more of. Besides Magneto & the twins themselves, we also had Magneto's relationships w/Luna (and by proxy, Crystal & the Inhumans) and w/Wiccan & Speed, Lorna's relationship w/Wanda & Pietro (PAD had just done a great story with Wanda & Lorna bonding as sisters), etc.

Time passes and life maneuvers.