DC hasn't really had that long a career in MMA honestly. I'm guessing you're counting his amateur wrestling career and that wear and tear of the training that goes with it? Not exactly the same as MMA, but still significant I agree. The weight cutting still there, putting his body through the wringer in training, etc.
But he did kind of get started late in MMA like Couture did. I think his chin might still be as solid as anyone's even at his age since he just hasn't taken THAT many shots like other guys did. Fight with Gus was a war, Rumble clipped him good in the first fight, but other than that he really hasn't taken much punishment at all. Jones beat him up some, but wasn't brutal or anything.
That said, Rumble's power is just on another level. I honestly think he might be the hardest hitter in the history of MMA, even including natural HW's. The fact that Rumble actually fought at 170 at one point is one of mankind's great mysteries. He really wasted the first part of his career trying to force himself down to weight that he shouldn't have been at. I mean, look at his fight with Koscheck. Can you imagine the best version of each guy in an openweight fight? Rumble at his size now would knock Koscheck (any version) into space.
I like Rumble rd 1 and KO honestly. Like I said, DC's chin is as solid as they come, but this time I don't think he survives if he gets hit.