Via: Contact Music "In a way, both John and I ruined our careers by getting together, though we weren't aware of it at the time. My initial reaction to rock music was, 'Oh dear, how simple can you get?' At first I thought John would carry on with The Beatles and I would do my own things, but he felt it wasn't right that we were working separately, that the union we had might not last, because of the pressure of the world."
Seriously though, even though the Yoko haters are gonna jump all over this, it's not really a fair assessment. I think it's pretty obvious to most that John was beginning to outgrow his Beatles stage before Yoko even entered the equation. He might not have gone quite as far left as he eventually did, but The Beatles weren't going to last either way.
You can not blame someone for following their heart.