That's true, but some of that resentment would have happened anyway. Yoko's presence would have happened whether or not Brian died. Paul was already taking over musically by the time of Sgt. Pepper, which is before Brian died. And Magical Mystery Tour was another Paul idea, also already planned before Brian's death.

I think Brian would have done a better job managing Let It Be, but by that point, the distance between the Beatles had set in. John, Paul, and George had literally been playing together since George was fifteen. They were childhood friends, and now they were all approaching thirty, all four of them married, and three with kids. Their personal lives were pulling them away from the band in separate directions.

Best case scenario, they may have lasted a couple years longer than they did, but if John had still fallen for Yoko and then moved to the U.S. (and then get stuck there because of visa issues), it's hard to see how they would have continued on much longer than 1971, whatever happened with Brian.