What band or entertainer was great until they sold out and went pop ???

I would add REO Speedwagon. In the early 80s they got a bit syrupy. Journey is also a totally different band from their first album and then their later Pop.
Edit: Also Rush starting with Tom Sawyer.

Aerosmith was once a cool grity blues/rock band....they went kind pop ballad hits.

Chicago

more than I can count...🤔

Love Cheap Trick but the 1st album outshines the rest I think

weren't genesis more progressive rock before phil collins joined?

Weird al

UB40 were brilliant until they turned into a karaoke act.

Michael Bolton?

Cold Play