Let's just look at facts...
Fitz has never been a speed WR, and as he has gotten older, he has not gotten any faster.
Fitz struggles to gain seperation with speed, and has to use his size/route running to try and create seperation.
CBs feel like they can play off Fitz, and just squat on underneath routes. That makes it tough for him to play outside because there are less route options for him.
Fitz has a huge contract next year, and as it is right now, you have to think the Cardinals will just move on if he doesn't make a decision to take less money.
This all being said, Fitz still have the size to be a red zone threat, has great hands, and draws coverage. Those are assets to have.
In the end, the ball will be in his court next offseason.