Ratchet & Clank is their safety net. For minimal effort, there will still be a dedicated fanbase who pick up the games. They're just treading water with the franchise until the next 'main' game, and using the time to experiment.
Insomniac were, throughout the PS3's life, one of the most productive studios for Sony. In the same time period that Naughty Dog made Uncharted 1 + 2, Insomniac had made two full-length Ratchet games, two full-length Resistance games and a shorter-length Ratchet game. They've always been running a pretty big roster with the ability to make two games at once. They just split the Burbank team up and made the North Carolina Studio, who made the last two Ratchet games.
I'd imagine it's much safer for them to make more Ratchet than to take a gamble on another new IP (Fuse is enough of a gamble as it is), or continue making Resistance titles which were very much on the decline.
I honestly don't believe they'll stop making Ratchet. Or at the very least, we'll see a proper PS4 entry. They seem very attached to the franchise, Sony seem happy to have him front and centre as a mascot, so it works for everyone. And with two teams, they can afford to do this.







