A lot of people would barely listen to Pet Sounds if it was only in drab, limited mono. I know that's difficult for previous generations to accept, but it's true.
I think that's a pretty gross exaggeration. I would imagine some younger folks listening to stuff on Spotify might not even know what a "mono" versus "stereo" mix is.
It's not like "Help Me Rhonda" or "Good Vibrations" have dropped off the face of the Earth because they were only available in mono.
Certainly, stereo remixes give an extra marketing opportunity, and whatever radio programmers left playing BB songs will often snag the stereo remixes for their playlists. Some stuff gets a lot more breathing room via stereo remixes (the vocals on Pet Sounds is one obvious example), while other stuff sounds murky and muddy on the multitracks (the backing track to H&V, "Do You Like Worms", etc.) and a stereo remix won't bring a ton of extra clarity.
But "Smile" is such a niche (a LARGE niche, but a relative niche) item, it's not as if there's a bunch of people out there that would have *for sure* purchased the 5-disc boxed set, but decided against it because they heard the "album" iteration was in mono.