I've seen this optimization. It makes more sense for the airline to just have free checked bags and charge a premium for a carry on. I think one issue is that they don't want the commotion at the gate check. I saw an airport pre-screener once tell a guy his carry on was too big and the dude was livid. This wasn't TSA and the guy wasn't an airline employee either.. I think he was just working for the airport. I would've been livid too.