obs, I'm sure The Beatles would indeed have embraced whatever was doing the rounds in the 70s, just as they did in the 60s. They were unashamed jackdaws who pinched a bit from here, borrowed a bit from there, so receptive to the climate they found themselves in and so eclectic in their cherry-picking.
That, I feel sure, would have continued in the 70s...but the main thing I think you're overlooking is that the 70s would have been much different - I think for the better - had The Beatles stayed together. Don't forget they always returned tenfold what they drew from their peers. They would I believe unarguably have remained a (perhaps still THE) major driving force on the music scene. And their instinctive quality control, as we all know, was second to no one's.
So I really do think that you wouldn't be regarding the 70s as so naff had a united Beatles been a part of it. A Beatles influenced/driven 1970s would have been a cool decade (I still think it was great anyway, but you get what I'm saying).
I know you've pointed out before how much they needed the 60s and how everything just couldn't have dovetailed together so perfectly for them in any other decade. True. But their huge talents and awesome chemistry - IF it had manifested elsewhen - would, I am confident, have registered hugely on some sort of level, they really were that good. And I will always maintain that they rescued the 60s from being as crap as the 50s (let's face it, there was some great 50s stuff but lots of tripe, as with any era really). So much of what made the 60s great would not have happened without the direct presence and influence of The Beatles.