I also think that the cessation of touring in 1966 was an inestimable factor leading to the B's breakup. They were a live performing band. They found their identity in playing together before audiences. They thrived on the feedback or lack thereof they received since the earliest of days. Now they were essentially only meeting in the studio to record in a mostly antiseptic environment with the most salient of their professional promoters privileged in be in their attendance.
Many of you who visit here are in bands or have been. Can you imagine not playing together in clubs, festivals, parties, etc. but only in a recording studio? The cohesiveness and the mutual camaraderie you share is felt primarily in the immediacy, tension and spirit of your live, unable-to-edit, performances together. The B's lost that precious component of who they were. They played hard, worked hard, in Hamburg and beyond, and now that experience was to be no more.
Is it any wonder they became increasingly solo oriented with Paul ultimately using the other B's as his sidemen, as John proclaimed and complained?
Maybe the most important root of their eventual end lies here and not in the most vaunted Yoko Ono disconnect. I don't think John could compete with Paul anymore anyway. I think he'd had it and didn't want to bother with it any further.
Paul was probably the biggest and finally, the last major Beatles fan. He tried to keep it together but the team was going down and they did and never returned.