I did watch the Goodall piece, and I agree that they pushed the boundaries of pop music, but again you'd be hard pressed to say that they ever expanded outside of pop.I guess that my point was that....
They 'did' stray from pop. They were the inventors of what became "the new pop", and the deviation is by virtue of them being the first to do it. The Beatles saved us from being drowned by both the Avant-garde and bubble-gum-pop.
It's like saying that Newton didn't stray from the commonly held beliefs concerning gravitation.... He did! And now we have entirely new commonly held beliefs concerning gravitation because of it. If your argument is analogous to "gravity didn't change, our understanding did" , then I concede.
Take a peek at the Goodall piece and see if you still disagree.
One example around composition and structure: Even in their most experimental songs, their drummer kept time (almost always 4/4). Also, the songs stayed within the acceptable, tried-and-true chord progressions. If you know anything about art music, you'd know that in the 1950s, musicians (across genres) were experimenting with freeing music from traditional rhythm and harmony.
Most of the Beatles innovations were using studio technology, or some pretty cool "accidental sounds" like the reverberating glass bottle.
As it relates to to the thread, I had just used an anecdotal experience where someone had called a song rigid. That song happened to be a Beatles song. My point was that pop music is intended to use repeated rhythms to create a beat, and the song usually sticks with the rhythm, with some minor variations. Again, I can't seem to think of many Beatles songs that vary from a repeated rhythm or beat.
My point was that pop music is often meant to have strict rhythms. Pop music is meant for mass-appeal and/or dancing, so the rhythm is important. It's the same with Afro-Cuban music. If you are playing a Rumba, you need to play the 2:3 or 3:2 clave specific to that rumba, and there is no room to bend time.