Without John and Paul effectively killing the practice of using outside song writers, the music scene would be much improved.
So basically, what you're saying is that having pre-manufactured, record label made bands or pop acts would significantly make for a better musical atmosphere? Not only am I glad that the Beatles didn't rely on outside songwriters, I'm glad that ideal spread throughout the music industry (for a while anyways). If it wasn't for that, the late 60's and 70's would be a very musically boring time.
As for no possible British invasion, unless importing old American C&W or R&B records to the UK was suddenly made illegal, I'm sure some other band like The Who or Cream would've picked up the slack if there was no Beatles/Rolling Stones based British Invasion. There were just too many factors in favor of such.