I'm not much of a fan of anything Paul did post Beatles though I only know a small percentage of what he recorded.
You could compile a greatest hits of the 70s from John and George and blow anything the Beatles did as a group out of the water in terms of sheer # of great songs. I think those two, especially Lennon, kept Paul from going too light hearted and goofy. Paul kept Lennon a little more grounded and his music a little more consistently listenable since he couldn't let Paul be the only one producing #1 hits.. Keep them all together for another decade and I'm pretty sure they would have kept doing great things as a group.
I think they still would have split up by the early 70's, regardless. Just too many creative differences. But there's no question John and Paul were better in a group than outside of one. John didn't really have many good songs post Imagine (1971) and Paul left to his own devices, was a lightweight, indeed.