It's not even a question of variety of gameplay types (the difference between your Killzone, Call of Duty, and Halo), but even if there were only two FPS series like there were only two major music-instrument games, they could last longer, because its a question of content. You could make as many Halo games as Dynasty Warrior games, and as long as you incorporated a different story and different weapons, you would have something at least halfway compelling
The Music itself was the content for the music games, and DLC, while convenient for the consumers, killed the genre in the long run, moreso in Rock Band's case. They either should have just made it a dedicated service platform rather than a sequel retail title, or not delivered any DLC at all, and there's only so much music out there worth putting in, as some of the stretches the later Guitar Hero games had to come up with will demonstrate
Monster Hunter: pissing me off since 2010.
