It's inconceivable because the Beatles were hugely talented with two and a half very prolific song writers all playing off each other, they had enough money to stop touring and to live in the music studio, and they hired outside talent considered the "fifth Beatle". Not to mention that experiment along with the pace did not last.
Source? I think you'd also need to account for how many big acts had song writing factories working for them back then. Lou Reed for example worked as an in house song writer for Pickwick Records before he went on to form the Velvet Underground.
