I do think the immediate major change outside of the Beatles would be Brian Wilson and the Beach Boys, and I do think that deserves further discussion. The reason Wilson gave up on Smile (which was heavily promoted and awaited) was that he heard what the Beatles ended up releasing, and felt the Beatles had already got "there" first. As has been said previously, while the Gap Album would not be something deficient, it wouldn't have been as far an evolution as Sgt Pepper, even if it were close to it. That may be just enough to keep Brian Wilson from giving up, allowing for Smile to be released. Then again, it didn't seem to take much to push Brian Wilson over his emotional and self confidence limit, so the Gap album may also be enough to do it since it will be a step up from what was already done with Revolver, which was a step up from Pet Sounds.
Even if you don't know much about the Beach Boys, if you know just their songs, you can tell that something went off track along the way. And that was when Brian Wilson mentally and emotionally collapsed. The more I look into it, the comparison I drew to the space race and the Beach Boys being the Russians comes off as more and more accurate. Whereas the Beatles had 3 major innovators (and Ringo), and were pushing forward and so far as I can recall did not receive major crushing opposition on their direction from anyone in authority, the Beach Boys were quite different. Brian Wilson was the leader of everything the Beach Boys were doing, and was the one pushing them forward, and they really were a mouth piece for his musical genius by a certain point. And along the way, the label was pushing back against him and the others began questioning him, and Mike Love especially questioned and put down what Brian did. The name of the album Pet Sounds comes from Mike Love saying "Who's going to listen to this shit? Dog ears?". (Nobody should like Mike Love. He's a bad human being. And all the Beach Boys, at one time or another, had a problem with Mike Love. Especially Dennis Wilson. Think if Ringo was a jerk with a giant ego and said he was a co-writer on all the songs and tried to take credit for what the Beatles did). That's where the album "Summer Days (And Summer Nights!!)" came from; Brian Wilson got push back from the previous album "Today" and they made him put out something that sounded more like what they expected (it was kind of subversive because Summer Days did push forward while hiding it under songs that sounded like what would have been expected). The people around Wilson seemed to actively pressure for the same old thing and what was expected, whereas even if the Beatles' audience did so as well, the Beatles pushed forward what they wanted to do and everyone ended up liking it regardless. That all came to a head around Smile, because the studio was pushing back on the time and money that was going into the album, and the other Beach Boys said they didn't like what they were hearing when they returned from an England tour and heard what Brian Wilson was doing.
After Brian Wilson gave up and collapsed mentally, it became Mike Love's band, and it was at that point that the Beach Boys became basically their own tribute act, playing their greatest surf hits and dressing up and acting like what they were expected to by the simplest stereotype. I've compared it before to the Beatles sticking together past 1970, and around 1972, putting on mop tops and teddy boy outfits and singing "She Loves You" and "I Wanna Hold Your Hand" on some Holiday Inn band tours. It's embarrassing. And you can see the limits of Mike Love's version of the Beach Boys in the lack of hits under his leadership, and the increasingly terrible albums. The only hit they had was "Kokomo", which wasn't written by Mike Love except for some lyrical change or two, and is a very soulless song for a banal middle aged listenership. I like "Kokomo", but that's what it is. And if that one song is the best thing you've done in decades and decades, it doesn't say a lot. That collapse came to a head with "Summer in Paradise", released in 1992, which sold less than 10,000 copies and was essentially Mike Love's album through and through.
That's a hell of a sad path for a band which was hanging in with and evolving alongside of the Beatles and the Rolling Stones and Bob Dylan, and was America's version of the Beatles. So anything that could keep them Brian Wilson's group for longer, or even butterfly away what Mike Love did, is very much a positive.
(There's also something to be said about a what-if concerning Mike Love being kicked out of the band).