Sam Darnold has over 18.8 million guaranteed this year. Now that salary is mostly wasted.
I can't imagine that Mayfield is a big enough improvement to make the Panthers a contender over the next couple of seasons, and with Darnold's salary hanging over them I can't imagine they would be able to sign enough talent to make a huge difference.
If Watson has to sit out this season it may be a blessing in disguise for the Browns. They'd suck bad enough to get a high draft pick, and have Watson as a QB with better talent around him in 2023.
IMO the Browns won this trade.