There's only so many people in the world who can play QB at a high level.
There's a reason that certain guys, given every chance, still can't rise above being a back-up (even a good back up).

And beyond that, QBs have to fit systems (or vice versa). They are not one size fits all, which too many teams subscribe to.

I think the Rams are in very good shape QB wise. If Goff goes down, they have a guy who has won a lot of games in this league against top competition. And the Wolford kid behind him sure seems to be a gamer and got an awful lot of playing time during the preseason.

QB is so complex today, as much because of complex defenses as anything else. So there's little choice but to give the lion's share of preparation to the starter. And then pray if he goes down that the guy behind him can play OK given the same amount of game week preparation time.