The cost of a replacement is very relavant to the discussion. What you're proposing is saving $5-10 million off a $145 million dollar salary cap to downgrade at the most important position being worth it. For a team that will not have salary cap issues in the offseason due to Suh going to Miami. Lions will have more than enough cap room to be a player in FA, so essentially you're watching Ford's pockets. Martha's straight, I promise.
I am talking about moving on from a guy that isn't what you expected him to be or what you are paying him to be. If the squad was making the playoffs or the offense was beastly year in and year out then it would be one thing but that ain't the case so yeah it's time to move on from that failure.