1st round was undeniably Cormier's with the big slam takedown & the fact he kept him there & roughed Stipe up. The 2nd round however was extremely close, they were both standing in front of each other giving as good as they got, I'd have given the 2nd to Stipe though on the basis that he took him down, albeit briefly, defended 2 shots & had DC in the more dominant grappling positions.
Saying he was being destroyed is an incredible leap, you should watch the fights without Rogan, I'd be interested to see the punch stats in particular for that 2nd round because with Rogan's commentary he made it seem like DC was outlanding him 2-3 to 1 in every exchange on the feet with crazy combinations & to me it looked like they were each giving as good as they got.
You mentioned DC's 40, you do know Stipe's 36? He's not exactly a young man himself and he's a fair bit bigger.. and pushing a high tempo is like DC's game.. even when he's 50 DC would probably still be able to push a lot of the HW's to their limit.