Sure, why not? I mean, the guy could die tomorrow of a coronary, he wouldn't have any peak years then. Or there could be a personal tragedy or major off-field distraction or he could just plain "lose his touch." None of these things are likely, but one thing anyone who follows baseball knows is that you better hedge your bets at least a little or the unforseen will bite you in the butt.
Barring something unforseen and unreasonable, though, no reason he won't have a couple more great years in him. There are plenty of players built more or less like Ortiz is built who are able to be productive into their late 30's.
Oh and, I didn't call for the Sox to sign Ortiz per se, what I said IIRC was that there was no particular reason to believe that the end of this contract was the end of Ortiz in Boston -- and there isn't.
Seriously, though, why even play the semantic game over something that small? There was no reason to even attempt to call that out. My statement stands on its own as a reasonable statement -- if I'd spoken in guarantees you would have called on that louder and been more right. So what's the point of this?