What if, when Stuart Sutcliffe left The Beatles …

Funkfingers

… to concentrate on Art and Astrid, Paul had stayed on guitar/piano and either John or George had switched to bass guitar?

How good, bad or indifferent would the band have been? 

How long would it have lasted?

ICBM

I could believe George might have done it, and been quite good - John just had the wrong kind of rhythmic style to do well on bass. They all thought George was the best guitarist though, so without actually replacing Stuart there was only one choice...

Tannin

Easy. Get Paul to play drums and put Ringo on bass. Fixed.

(Anyone can play bass. If bass playing was hard, bass players would not be able to do it.)

Funkfingers

Pete Best on bass, huh? (Barry Wom was yet to be discovered, hiding in the back of the van.) 

Tannin said:

Anyone can play bass. If bass playing was hard, bass players would not be able to do it.

Them's fightin' words, Buster. 

If Paul McCartney did not exist, it would be necessary to invent him … and James Jamerson and Jack Bruce and Larry Graham and Jaco Pastorius. 

Neill

The roles weren't that clear cut anyway.  For example there's a lot of debate as to who played what on "Paperback Writer", George certainly played bass on one of the recording sessions, I believe, and I'm pretty sure Paul was responsible for the intro guitar riff.

How the fab four lined up on stage is a complete distraction from the way in which they approached their music and recording in the studio.