IIRC, the original Days of Future Past made it pretty clear that groups like the Avengers fought and lost against the Sentinel onslaught. Gravestone markers existed for members of the Avengers, the FF and some other groups in background shots, I think. Specific reference was made that the Sentinels killed off most superheroes because they didn't conform closely enough to the 'standard human template' and were in the Sentinels eyes, just as much mutants as the X-men.