I can easily see Messi transitioning to a deeper-lying role or even a number 10-ish role that doesn't rely on mobility very well. It's very possible we've already seen the best Messi has to offer, but I think he'll age much better than Ronaldo, who's game is so heavily based on sheer physicality, out-muscling and out-pacing the opposition.
Messi will get another legitimate shot or two at this thing, I have no doubt about that.
True but Messi relies on pacing a lot too. But if there's any player that can change their play style to suit their age it's those two though.