Don't knock Lego Rock Band. Lego Rock Band appeals to my kids where as RB2 doesn't appeal to them at all, so LRB, for me, saved the genre. I don't care to play the games by myself much, and so having a version that my kids like makes a huge difference. Plus I like that I don't have to first go through all the songs and figure out which ones are kid appropriate (though they go overboard in rejecting songs, perhaps, but there's enough songs anyway.)
Where Harmonix went wrong, I think, was with pricing Beatles Rock Band higher then past versions, especially the bundles. I think that killed any momentum the RB series had. Sure, the Beatles are great, but paying too much for them isn't.