He only turns 31 this September. Has only had 6 full seasons in the majors... He's far from "past his prime." And, since he's been in the bigs, he's routinely been considered to be at the top-tier of defensive firstbasemen. And if you were to speak to his coaches and teammates, they would tell you he is an incredible leader.
He is off to an ugly start for sure...and man, he has looked horrific at the plate for a large % of his at-bats... But even when he is has been hot, he still had those at-bats where he looked completely lost. He's very much a "guesser" at the plate and if he's guessing wrong, it usually doesn't look too pretty...
As for the big contract - I can assure you doesn't lose even a second of sleep over the $$... It literally means nothing to him. He'll receive a very nice contract at some point in the next 6-12 months. I hope it's with the Dbacks - and so does he. But, it will come down to so many other dynamics... Can the team effectively off-load Greinke? Gotta keep Pollack. Gotta keep Corbin. Gotta extend Archie... Can't do all of that and keep both Greinke and Goldy.