Some would simply call it an evolution of style, and others might call it a sellout. I'd like to hear your examples of musical acts that changed so radically over time that you'd consider it a sellout.
I started to type out a huge, involved comment, but I don’t want to come across like mirror-world Patrick Bateman, so I’ll just say here: Genesis. 1970-1976 and 1978-1998 were almost like two completely different bands.
I think it is fine to just treat them like two completely different bands. Personally, I like both of them. But if I had to pick one 70-76 of course would be the choice.
It's taken me years to regain an appreciation for their post-Duke catalog. I was intensely negative on anything later for close to 15 years. Slowly I've been able to rediscover what it was about their later output that I initially liked, and maturity has smoothed out some of my own rough edges as well. I still find We Can't Dance to be unbelievably limp and lifeless, though.
1978 marked the departure of Steve Hackett, probably their proggiest songwriter. There's still a bit of old Genesis magic in the following 2 records but by 1981 it was basically completely gone
I enjoy Coldplay as much as most people don't enjoy Coldplay, which is quite a lot considering what I've heard some people say. They have, how shall I put this, "refined their sound" extensively over the past decade. Parachutes, A Rush Of Blood To The Head, X&Y, Viva La Vida: that's a remarkably solid group of albums. And then there's been a string of albums that I admit I enjoy rather less than those. Those younger than me still seem to enjoy them, so they must still be doing something right. So you can call that what you will. I wouldn't call it selling out per se but they certainly have taken a direct route to commercial success and aren't afraid to change with the times.
I'm not sure this is exactly what you're looking for, but it literally a "sell-out" and has always bothered me. Jim Morrison never wanted his music used for any other purpose, but the rest of the band went behind his back and sold a song (I think to Ford) for advertising.
Then there's Bob Dylan. Never literally promised not to do advertising, but "Song To Woody," on his very first album comes about as close to a promise as you can get. And then selling his entire song catalog? Come on!
I think selling his catalog is a smart move. It cements his financial stability for the rest of his life, and probably his children as well. But yes, once he's sold it, they can do whatever they want with it.
But it was so meaningless a gesture! It would have been trivial for him to remain true to his own artistic vision. If his message as an artist had changed (as it did many times) that would be different. But shilling for Victoria's Secret was not art. He just traded on his "brand," and that is truly selling out.
Not sure if I'd call it selling out specifically, but I've hated every single song Weezer has made since the white album. Just not a fan of the way they've taken their music
For something a bit more modern, Fall Out Boy. From Under the Cork Tree was their last decent album imo and even that was a shift from Grand Theft Autumn. Hearing them now they sound like generic pop nothing to my ears. Such a shame.
Metallica is the example that most-readily comes to my mind. They went from being very pro-tape-trading in their early days (because they recognized that open access to music = more fans) to whining "Nyapster stole our myoosic :("
I like this example because it's not about their music, it's about their attitude and forgetting where they came from. Personally, my favorite Metallica album was Kill Em All, and I haven't really liked much of what they did after that.
I liked everything up until the black album. The black album was hit-or-miss for me; it had some parts i liked and some parts that weren't really doing it for me. Load and Reload sounded (to me) kind of generic; it was metal but it wasn't up to the standard I expected out of Metallica. The less said about everything after those albums, the better.