Palmer was average this year, but IMO can improve to elite next year. He can't play worse than he did this year. New personnel, new system, new coaches, new line, new schemes and new formations. I also don't believe it's accurate to say he can only decline at this stage of his career due to his age. Our team next year will be, by far in my opinion, the best team he's ever been on, on both sides of the ball.
He can improve his TO ratio compared to when he was in Cinci (let's face it, Chad Johnson wasn't an elite route runner--he couldn't even get on the field in NE when they desperately needed receivers), Oakland (where Fitz says Palmer had to throw to himself), and the 2013 Cardinals (with a hobbled Fitz, a lackluster Roberts, a 2nd-year player in Floyd, and hands of stone Housler, all while learning a new offense behind a bad offensive line).
He doesn't even need to improve his game or mechanics necessarily, all he has to do is minimize his mistakes on a vastly superior team than any he's been on. He knows he's on his last ride, taking his last climb up the hill, all he has to do is hold on.