Let me say up front that these thoughts are my opinion and I really don't want to get into a pi**ing contest with anybody so flamers look elsewhere.
To really understand the impact of the Beatles it helps immensely to have lived through it. There was a ton of social unrest, the Vietnam War, civil rights, cold war et.al and the Beatles timing was, IMO, greater than their music of which I am a huge fan. No apologies offered for that. The Beatles offered an alternative to troubled times and problems. Especially to us younger (at that time) folks who knew something special was going on. The Beatles were huge and commanded a lot of attention. Once the "love song" (Help!) era ended they began (with Rubber Soul) to release albums with strong social commentary and everybody paid attention; kids, parents, teachers, media, artists, politicians, etc.
Are they responsible for the existence of modern music? Of course not. However, the influences from which they drew (and most of them have been mentioned here) inspiration coupled with an unbridled enthusiasm (and heavy medication at times) broke down many barriers for other performers both literally and figuratively.
To me the litmus test of greatness is longevity. Bach, Mozart, Cole Porter, Duke Ellington, Harold Arlen, Bob Dylan, etc. Will the Beatles pass the test of time? Probably but it's much too early to definitively say yes.
Have they greatly influenced rock music? Absolutely. However, even without the Beatles rock music most certainly would would have evolved into something interesting. Youth must be served.
Goo Goo G'Joob