I think their point is that it's best to bring him in on a short-term contract, as players can decline sharply in mid-thirties and - while he's still brilliant and full of energy now - that might not be the case post World Cup.
I don't think anyone thinks he won't be a top player for the next two years at least. I certainly think he has a lot to contribute for the next two years at least. It's just after that when you get to 36 and over it's really getting very old in footballer terms (yes even in this era).