Not that anyone's saying it here, but we do have to be careful about slapping the "Sellout" label on bands who consciously try to do what fans want. Iron Maiden has been mentioned (without the label), and AC/DC is another band I hear about all the time, how every album sounds the same. There's nothing inherently wrong with doing the same thing over and over, if it's what the fans want and what the band like doing. Sure, the band is aware that they're doing exactly what the fans want, but it's not necessarily because they want to keep making tons of money. Some bands do it for the sheer joy of giving people what they want. They know their fans love what they do, so they just keep doing it. Are they "artists"? Are they "sellouts"? Maybe and maybe.
I know it's not exactly the same thing, but since I'm an old fogey, I think of bands like Chicago. They still record albums, which sound nothing like what the original lineup created. Then they go and tour, and they're psyched about their new album and play some of the new songs. Except no one actually knows the new songs; they want to hear "25 or 6 to 4" and "Saturday in the Park". Songs that are 40 years old. So of course they play them. Why? Because they know it's what the fans want.
I even hear people say, cynically, "It's been a few years, they must need the money." No way. Those guys were millionaires a long time ago and could have retired years ago. Some of them have. But they go out and tour and play the same songs again, because they're musicians and that's what they do, and the fans love it and keep buying tickets, so why not? It's a symbiotic relationship.
tl;dr version: It's not always bad to give the fans exactly what they want, and not always wrong of the fans to expect the band to give it to them.