I don't think we win that Super Bowl with Goff instead of Stafford, so the trade was a success. Stafford was lights out. Non-stop come from behind wins and game winning drives. Goff likely could have done it too in some games but not in all of them.
I also don't think Goff develops under McVay the way he did under Campbell. McVay didn't trust him to read a defence. If that was right or wrong, up for debate, but it stunted his development in that part of game, he was never going to grow with Sean keeping the training wheels on.