Backup QB is overrated. You need a guy that can finish a game without blowing a 10 point lead but there's just no room in the salary cap to prioritize a second QB on the roster that is capable of winning consistently. It really falls on the coach to make it work while the starting QB is hurt, not put it on a QB to be great. Unless, of course, you want to spend premium draft capital or pay $6-8m for a good veteran backup.
Put it this way, do you want to draft Drew Lock in the second round and pass up Elgton Jenkins? Not perfect hypothetical since Lock went before, just pretend for a second. Another scenario - would you cut Bulaga and use his $8m cap savings to sign Teddy Bridgewater? That's kind of the choice when it comes to having a good backup - use premium draft capital to get a young guy and develop him, or pay for a veteran backup.