Getting rid of your best player on a team that envisions itself a Super Bowl contender is colossally stupid.
Not only does it make your team worse in the short-term, it presents yourself as an organization that is difficult to do business with, especially for premiere players, making contracting harder in the future. The one thing the Cowboys have always reliably done is pay their homegrown talent, just zero reason to crater that with someone as polished as Martin.
Just absolutely 0 upside to this.