I heard an excellent breakdown this morning of Jimmy G. He has everything EXCEPT the arm to stretch the field horizontally. You can loft rainbows deep down field to make up for a so-so arm. You can't do that when throwing outside the numbers. So the way to protect against the Niners' passing game is to protect inside the numbers and force JG to throw wide.
Jared Goff simply is not a good enough player to run the playbook McVay wants. Enter Stafford who is. It really is that simple.
Both are good, starting quality NFL QBs. But... they have limitations that lower the team's possibility of winning a Superbowl.