Personally I think Cassel is a very high character person with a great work ethic .. He would do anything to help this team out and to get better.
Unfortunately he just can't get the job done ... I believe if Haley and Pioli hadn't insisted he play every week and would have let him sit and learn or even pulled him during a few of the bad weeks (both for him or for the team in general) and let him learn he would have been a great backup and possibly a good starter. Unfortunately when you pay Lamborghini money for a Chevy it is hard to leave it in the garage.
On the plus side Cassel has made enough money that he and his family should be able to live comfortably the rest of his life and I would imagine he has the skills to do other things in the community / world where if he needed income he could make a reasonable income somewhere else such as running a family business and I believe he would make a great coach at some level ( positional / coordinator or with experience even head coach or assistant.) I just don't want him to suit up as a QB for the Chiefs again.