Brutal Legend tanked because nobody gives a fuck about Jack Black these days. 5 years ago, this game might have done a bit better, around the time School of Rock came out. Also, just because it has a lot of homages to metal doesn't mean people want to play an RTS game. It is a Tim Schafer game, where his stuff is vastly overrated and appeals to a very vocal niche. Publishers in the past have incorrectly assumed that internet hype from vocal minorities = gangbuster sales.
It'll probably do 500,000. Not that bad but EA can't be too happy. Time for Timmy to find some other unsuspecting publisher to sucker.