Unless your back-up is a young guy being groomed to eventually become the starter, a back-up QB is like the value of a fire extinguisher. You need one only because there MIGHT be an emergency and God help you if you don't have one. But it only has one purpose....to put out a fire in a rather messy way.
When you have an elite level QB like Stafford, no QB with real potential as a starting QB WANTS to be behind him because he has no chance to become the permanent starter even if the starter is hurt for several games and the back-up performs well. So, all that is available is guys who want a paycheck, want to stay in the game, but don't have too much chance of becoming a starter. There are those exceptions like Mayfield, of course. But he is an outlier. There is no Mayfield II sitting out there.
Signing JG means the Rams have no one on their radar to supplant Stafford in the near future. Should they draft a QB anywhere in the first 3 rounds, they'd have to keep him on the 53 or risk losing him. They do not WANT 3 QBs on the 53. Hence, they're not going to draft anyone that they cannot safely put on the PS.
As long as Stafford is around, and McVay is around, we're going to see someone like JG as the back-up.
BTW: I don't think McVay is a QB developer. He's too impatient. When the time comes for Matthew to hang'em up, the Rams will go all-in to acquire a good vet QB who is already developed. McVay's role will only be to tweak the QB's game a bit, and to build an offense around him that works best for that QB's talents.
