Probably Harry Nilsson.
A close second might be Gerry Rafferty.
Would they accept? I don't see why not.
Just me, I would not have noticed at all.If it was up to you to hire someone to step into his musical, vocal, and songwriting shoes, who do you think would have been up to the job, and how likely would they have been to accept it?
In a parallel universe I'd say Muff Winwood. After Macca's mysterious disappearance, Winwood joined in '67, set a direction and kept things together. The band kept recording until 1976, and the only real mistake was the 1974 quasi-spiritual concept album/musical Weeping Prophet they made in collab with Chi Coltrane.
Brian Wilson....if only to sink the Beach Boys at their peak & spare the world of having to deal w/ the insufferable Mike Love any longer than necessary plus we never had to see John Stamos on stage.