Good post. power is often the last thing to go, but I thought he was about to knock the iron chinned Marquez out (who I think was on roids or at least some form of PED's) right before he got ko'd himself. He also had Mosley badly hurt who had never been stopped until his last fight with back spasms. His power is still there, but he moved up in weight and he's been fighting elite guys. It's always harder to knock those types of guys out. That's consistent throughout boxing.
But yeah more often than not guys have trouble bringing concussions up with them through the weight classes. I think in Pacquiao's case age is playing a bigger part more than anything else.
Having power is one thing, being able to maximize that on someone, preferably when they don't see it coming, becomes much harder to do with age. I wouldn't say that really lose their power more the ability to deliver it as effectively as they once did.