There was a general consensus before The Beatles that guitar music/rock and roll was just a "fad." however, by the time they came along, it was largely accepted that guitar driven music was here to stay.

The Beatles were not taken wholly seriously at first because they played what was called (in Britain) "skiffle," which essentially translates to harmless boy band pop party music. I wanna hold your hand is a good example. It's very well done for teen pop, but at the end of the day, its just a very good teen pop song. If the Beatles hadn't ever undergone a huge transformation in style, they might be looked at like N Sync- just a very good boy band that sold a lot of records and had a few members be successful commercially afterwards. (This is an exaggeration, but one made to make a point. Lennon and Harrison would have become famous doing something else regardless in my opinion, but that's just speculation)

I think critics as a whole expected for Jazz to stay the most popular style of music, along with blues based tunes like the Stones were doing. And they were largely right until the Beatles made their transformation to more complex music and the fanbase followed them.