It is not that DDP was tired in championship round, but Strickland looked more fresh. Ive noticed he has good tank in late rounds agains Adesanya. In general, I like main event fight. I think Strickland landed more strikes. That would be enough to win imho, but it was DDP who welcomed each strike with face.
Strickland looking fresh in the last rounds is proof he could've been more active in the earlier rounds. I think even his corner told him to get more active.
As for DDP, i think he's one of the fighters who would breathe through their mouths when getting even a little bit tired. That makes him looked way more exhausted than he actually is.
Looking at the number of strikes alone could be deceptive. He could have built that advantage in the 2 rounds won but still lost the other 3 rounds. Plus, the actual damage caused is what really matters.
Who faces DDP now? Izzy?
If Adesnaya is to return, this would be the perfect time. If not Adesanya, it'd have to be either Cannonier, a rematch with Strickland, Chimaev (when he recovers from whatever health problems he is having) although he doesn't deserve the title shot yet, or Paolo Costa if he manages to defeat Whittaker.
P.S. Feel sad for Strickland, as I think hype around him going to go really low, but he is such a funny guy 
Why though? The hype around him was built on his personality, not on his skills etc. He's not going anywhere and he's got nothing to be ashamed of. He lost by solid decision against a guy who was not only undefeated in the UFC, but who also won all his fights (apart from one) by finishes.