Sutcliffe was an artist...not a musician, per se.
AFAIK, he left the group because he wanted to-
a)Be with his girlfriend in Germany
b)Get serious with his art
I didn't see
Backbeat. Wasn't this stuff featured in that movie?
I know there are some reports that Paul forced him out so he could become "The Beatles' bass player". Rubbish(from everything I've read).
I don't see a problem with Paul being 'forced to be a bass player", either. Electric bass was still relatively new...many soon-to-be-great bassists of that era started on guitar(Joe Osborn, Carol Kaye, etc)
Too, IIRC, at the time, Paul didn't even have a guitar & was playing whatever piano the clubs happened to have on-hand. Harrison & Lennon already owned & were playing guitar...why have a 3rd guitarist(unless you wanna be Black Oak Arkansas or sumthin)?!?