Most working actors in show business would love to have Rocky's problem. He's doing just fine as an action star, and his kid-friendly roles are almost certainly part of the plan, not things he get's stuck doing because there's nothing better for him.
I admit, I don't watch a lot of movies. And I hope this doesn't come off as sarcastic, but can you point out some of his action movies that are actually good, and feature decent fight choreography?
Arnold could get away with his size in the 80s, because back then most action movies and scenes within those movies revolved around guns. I've seen pretty much every Arnold movie, and I don't think he has one single hand-to-hand fight scene which isn't laughably bad by modern standards.
It's a shame, because the Rock has amazing physical acting skills. His fake punches while wrestling were some of the best looking among the main event scene at the time. He can sell, he can bump, and he is (was) surprisingly agile for a 6'4 monster.
Perhaps part of the problem with casting Rocky in a highly physical, action oriented movie is simply finding suitable stunt doubles. That was also a big problem for Arnold in his early career...
EDIT: I forgot that Dwayne has been in some of the recent Fast & Furious movies, but I haven't seen any of them except the original. So maybe he's got some good scenes in those movies, and I just haven't seen them