It seems the Beatles sure thought it was revolutionary. They were scared of him. Didn't competition with Pet Sounds result in Revolver? And didn't subsequently that experience, plus rumors of Smile and the release of Good Vibrations, result in the pull-out-all-the-stops effort that created Sgt. Pepper's?
I understand you're underwhelmed by that album, but it seems the Beatles were driven to make what's often considered to be their best work because they were spooked by a single boy genius in LA.
You can come at the music of Brian Wilson from many angles, I enjoy the harmonic view. The music still holds up on that account because music today is regressive, we have deconstructed the same tired plagerized african-american grooves over and over to the point where there are no strong or weak beats, nor even tones to map tension and resolution around, all just to crank out cheep schlock crap to pollute the airwaves with. It would be avant-garde if it wasn't so shallow and exploitative. The future sucks.
Yesterday's orchestration is just acoustic guitar + string quartet
https://faroutmagazine.co.uk/how-the-beatles-rubber-soul-ins...
I too went through the entire Beach Boys catalog a few years back and indeed you hear the progress of pop music from "California Girls" ditties (Paul teased with the Beatles "Back in the U.S.S.R.") to the more dare-I-say drug addled "Good Vibrations" (and more experimental "Heroes and Villains").
But as I sort of hint at, marijuana, psychedelic drugs could be just as responsible for the trajectory of pop music.
Go through the Beatles catalog and I would argue there is an even more dramatic shift in trajectory with the influences of Dylan, pot, later LSD. It's well known that the Beatles were the biggest influence on Brain Wilson's song writing — it could just as likely be then that The Beach Boys arc was simply following (anticipating?) the Beatles'.
The orchestration in Pet Sounds though follows the McCartney/Martin "Yesterday" by the Beatles as well as Phil Spector's production at the time.
I'm also on the fence regarding Pet Sounds. I'm more impressed by overlooked gems like the Zombies "Odessey and Oracle" than what may be the over-hyped albums like "Sgt. Pepper's" and "Pet Sounds".