Mitch is 25 and people are like give him 2 more years, maybe he'll become average.
Winston is 25 and currently on pace to become the 8th ever QB with 5000 passing yards in a season. He doesn't need to fix anything. Mitch's five 3 and outs a game turn the ball over more than 1 or 2 interceptions.
Production without wins is rather useless. For those who make the interceptions argument, you can see Eli and Favre for reference as INTs are not always the end all be all when it comes to overall value.
However, in Winston's case, his production has not truly translated to team success. It's a mixed bag. Combine that with his off the field issues since, I don't know, forever, then you get a sense of how risky he would be to pick up.
It's quite rare for any FA/Traded QB to bring a team a Super Bowl. You also do not see it translate to team success as most successful teams since the league went to 32 teams has been the result of developing a QB and never letting them leave (Rodgers, Roethlisberger, Wilson, Brady, Mannings).
81% of Superbowls in the last 16 years are won by QBs who were drafted by their team. The notable exceptions include Manning, Brees, and Foles.
If Wentz does not go down, you can argue that he likely also wins and makes the percentage even higher. The other two are GOATs.