Yes---Denny was good for the group, but I would guess that Macca would have done it all himself (he darn near did anyway), forged ahead and made McCartney II much earlier, and, dare I say, much better.
I agree with you there - for a start, it wouldn't have been synth-dominated! So many songs on Mac2, I just think, "could have been really good, IF ONLY . . ." *sigh*
The difference between genius and insanity is merely the difference between success and failure.