make interesting but slightly different records, sell a few
make less interesting and a bit more mainstream records, sell more
make uninteresting mainstream samey records, sell lots
make britains-got-the-next-ice-skating-x-factor-dance-talent, sell gazillions, but soul dies
See, where we can probably all identify acts where at least part of that trajectory is true, there’s plenty of others who don’t fit it all. Fundamentally, that assumes that selling truck loads of records = selling out and taking the creative ambition out, which is not always true.
How about ABBA? Pure pop with no pretensions to be anything other than mainstream, not my cup of tea, but nothing fundamentally wrong with the way they wrote and performed songs, and does the fact that lots of people liked what they did make it less good?
How about Radiohead? Their first album is probably their most mainstream-accessible and they’ve gone progressively less so as they’ve gone along. And continued to sell loads
Beach Boys – started out with simple, mainstream, unadventurous pop, then developed pretensions to be more artistic, made “Good Vibrations”, often cited as the perfect “pop song” and not because it lacks ambition, quite the opposite.
Bowie – only started selling records (with the exception of Space Oddity) when he stopped trying to do disposable jump-on-this-week’s-genre-bandwagon singles and became a bit more creatively ambitious.
So, in summary, your description of the trajectory applies sometimes, but does seem to be widely accepted as a universal rule, with the effect that there’s a received wisdom that anything that sells well is inevitably rubbish, and the artist is a sellout, which is frankly cobblers as a universal rule.
EDIT: Forgot about the Beatles – probably the exact inverse of your trajectory. Increasingly ambitious / pretentious* (delete as per your opinion) as they went along, selling stupidly huge amounts of records all the way.
