The original mix was mono, the stereo stuff is the equivalent of colorizing black and white movies, adding 3-D to a 2-D film or remixing a stereo album into surround sound.

Brian Wilson is apparently deaf in one ear, which would have been less of an issue with mono.

Phil Spector (who was an influence on Pet Sounds) and other producers of the era were aiming to get something that sounded good when played on a car stereo of the time over AM radio and that affected their approach. The Phil Spector boxset was called "Back to Mono".

I think there are professionals who care about recording quality, and then there is an esoteric pop culture holding subjective opinions on sound. Both are labeled audiophiles at times.