It's true enough that interest in music games has waned (compared to the quick and unreal boom it saw), but we would have not have continued to produce tracks for as long as we did if the business wasn't profitable. Regardless of the continued interest, the fact is that we are currently working on several new projects that require the full support of our developer resources to make the best projects possible. This isn't an easy decision to make, since we clearly loved this franchise, which is why (in my opinion) we've offered unprecedented support for the series since RB1 launched in 2007. 4,000+ pieces of additional DLC content, week after week of fresh content... that's CRAZY. Correct me if I'm wrong, but I don't believe you can point to any other console game that has received that kind of post-release support.
As for the statement that it's "easy" and as to why we "let amateurs do it," that's incredibly dismissive of the hard, dedicated work RBN authors do, and highlights the fact that you really don't know what it takes to create a fun, playable track for games like these. That's fine, most people don't. Add even MORE layers of production in our case: take into account not only the money it costs to license tracks (including tasking our kick-ass music ops department, lawyers, etc. to even make these deals), as well as the resources to submit (and have approved) the content to three different platform holders week in and week out.
But it's been absolutely worth it -- building a platform like Rock Band has been incredible, and the fact that there's still this great game with so much (and such varied) content available for playing is still exciting to me (and apparently other people, based on the pretty positive responses I've been seeing today).