You make a good point, Bobby Ball was almost 'tried out' as a Compo replacement, exactly like Keith Clifford, Tom Owen and Brian Murphy were.
However, I think Bill Owen was too established in the show to be directly replaced, yet it didn't stop Alvin moving in below Nora and acting in a similar way to Compo.
I think Tom was a reasonable replacement - after all the son often becomes the dad, and would move into his old home, and the over affection from Nora was good because it was the opposite to before - meaning fresh new ideas and comedy. Not many liked Tom in this capacity though, it seems.
Keith Clifford was good because he and his character never copied Bill Owen's Compo.
I don't think Bobby Ball would have made a good long term Compo replacement, too young, too different, and yet too similar, in a strange way. I myself dreaded him becoming a regular, which happened a lot with new characters in the latter half of the show. Lot of people liked his performance though.