The whole 70's-thing is kinda popular now though, just take Pain of Salvation's Road Salt, lots of new bands sounding more "70's", and the fact that bands like The Beatles are having new waves of remastered albums and such, personally I'd say the music from the 70's hasn't been this 'popular' since the 70's itself.
Lots of bands that has been doing music for a while goes back to a bit older influences, as much as newer bands forming their sound a bit after how it used to sound.
I'm not saying it's a bad thing or that Opeth are selling out, because I'm really intrigued by the changes they're making.
Going back to a 70's type of sound could be considered "selling out", at least in the terms of that music being more hip now then a few years ago, but in the case with Opeth, I don't think that will be the case.
We all know that Mikael is heavily influenced by a lot of obscure older music, Captain Beefheart for example, so I'm fairly sure the new music will have a more 'odd' or 'unique' vibe to it.