the notion that future timelines are automatically to be treated as alternate universes is a good one, but the current Avengers - Maestro storyline and some previous Kang, Doom and even Future Imperfect seem to be extending the 'future' into the 616 present.
I mean Kang works for Maestro, thus Hulk will become Maestro (😖) and in FF, 616 Franklin had to beg a younger Doom to save his grandfather's arse (but this led to some vague time-reset, so not quite sure).
basically, the characters, attributes and relationships are 616, just that their actions happen in a revised past or an existing future... this is quite different to the traditional "alternative universe" idea, where powers, persona, relationships are different, thus feats cannot be compared to extrapolated.