I think Baby Boomer/60's rock got a bigger pass on this but there's tipping point where a band/artist has to change a quantum amount to stay interesting but not too much to loose their fans.
Examples.
I don't think Oasis ever changed at all. They hit a Grand Slam on their first album and then because it was so successful, never felt the need to and they had a lot of drugs so, why bother?
Blur's first album was sort of baggy, then they did three mock-Kinks story albums, then an indie/Pavement-ish one (the one with "Song 2"), an out there, almost Spirtualized-one ("13") and then a world-beat influenced one. So they changed in all the ways Oasis didn't. Now, Blur aren't as popular as Oasis and Noel Gallagher for some reason can charge $100 for muso "Dear Prudence"/"All The Young Dudes" ripoffs but you have to evolve or die.
Also, I think the Black Keys can get away with it because of the authenticity/credibility issue. The Black Key have percevied authenticity/credibility. The White Stripes do. Radiohead does. They change their sound and it's welcome and accepted and makes their art stronger. But someone like Panic! At the Disco or......The Streets or someone. Even the Arctic Monkeys. They get cut less slack.
Brian