That won't be the sole reason, or main reason, why he'll fail in DEN -- and what are we really talking about here, really? Failure... means what? Not win a SB?
More important variables will be if they can build an entire OL/develop one that can keep him from sustaining some major shots at his advanced age, something he avoided many years in INdy, but as well... can they surround him with a nice set of weapons and a good ground game? Will the D continue to be very good? Can he continue his high level of play? With obvious age, comes deterioration and he has to avoid that and many other factors.
Look, its a success if he plays and they get sell outs, merchandising is great, and he plays for them for the remainder of the deal and plays decent ball.
It's not fair to expect vintage Manning, but it's fair to say that if they win a div. title or two and get a chance in the post-season, even if they don't ever make a SB, it won't be complete failure.