I think there is a lot about CJ's injury situation that Pete and the team haven't told us.
Ever since his fractured scapula in 2016, he's just not been able to stay healthy. He had some "abdominal surgery" this off-season. I think there was a long-term issue that simply didn't heal right, and the team and doctors involved somehow guessed wrong at different times about what he'd need to get to NFL-ready body health state. I'm reasonably optimistic that it's finally been corrected and we will see a healthy, dynamic, exciting CJ for much of this year. This year, he could easily be one of those unheralded X-factor players that makes several difference-making plays over the course of the season to get the Seahawks over the top again.
In the 2013-2014 glory years, there were several season-changing plays (in hindsight) over the course of the year, where you can point to a few plays those unheralded guys made that preserved a W. Last year, for example, Tre Flowers was one X-factor guy, rookie making an open field tackle on a shifty RB preventing a 3rd down conversion and making the W possible. OL Jordan Simmons was another, when healthy, filling in for Fluker and making us not even miss him, with big-number team rushing yards games. Dissly was going to be that X-factor guy before his injury.
You just don't casually get rid of an ace-in-the-hole X-factor guy like Prosise, especially not at a position like RB with such a high injury rate, and especially not when he's cheap to keep.