wouldn't have mattered if Paul Brown himself was coaching the O... Mac, when pressed, folds. He is not an innovator... That's not to say he can't play QB, he can, but when he is pressured he falls apart... His game is based on accuracy... when he has the time he can sit behind the line and find his guys and do well... but he's not going to put a team on his back and drag them across the line...
In SF with Shanahan, he has a coach who will sit there and micromanage every play telling Mac exactly what he wants... and as long as he stays within those bounds a majority of the time he will do well... He's a lesser talented Jared Goff in a sense... Last year in Jax is a prime example of this - Even with a very QB friendly coach, he reverted back to what he was doing here... had a better completion percentage than Lawrence, but had a 1:1 TD/Int ratio... you can't consistently win under those circumstances...
He can carve out a very long NFL career for himself... can make millions of dollars doing so... as a Back-Up...