Teams with starters still on their cheap rookie contracts can afford to spend more for a backup. If it is that young QBs first season as a starter, it would be prudent to have that quality backup in case he sh*ts the bed, assuming there are any pretentions to winning. Without such pretentions you might throw a Josh Allen or DeShone Kizer out there to take his lumps now for valuable experience later. Teams with franchise QBs in 2nd. contracts and beyond @ $25 mil, $30 mil and more per year do not have that luxury.
"Less than" 15 points vs. exactly 15 points is splitting hairs. Only one team in the last 10 season had exactly 15.0 points against, that being the 2010 Packers. We tend to forget how good that defense was, among the 10 best in the last decade. As for 20 points or less, that's a top 10 defense. I would expect a "good" backup with a good collection of talent to do better than 0.500 against average teams. If there isn't that good collection of talent that team isn't going anywhere regardless.
I'm sure if you put out a proverbial help wanted sign in front of Lambeau field saying, "Experienced NFL QB wanted for backup position, starting pay $7 mil," you'd have agents lining up around the proverial block. That doesn't mean their clients are worth that money. You'll be hard pressed to find a team with a veteran franchise QB making big bucks paired with a $7 mil backup.
The problem is that no team can cover all contingencies. Teams spending big bucks at QB have already sacrificed a couple of good starters at other positions compared to a team with a rookie contract starter, e.g., Rodgers' $26.5 mil 2019 cap cost vs. Mahomes at $4.5 mil, Goff at $8.9 mil, Wentz at $8.4, Watson at $3.8 mil, Prescott at $2.1 mil, Trubiski at $7.9 mil, Mayfield at $7.4 mil or Lamar Jackson $2.2 mil. I may have missed somebody worth mentioning.
So the question is whether you're willing to sacrifice a 3rd. good starter relative to that competition for a backup QB that may never play? The numbers say the answer to Dirty Harry's question, "Do you feel lucky, punk", must be, "Yeah, I do, because I have to". Then you hope that chamber is in fact empty.
Of course if the aim is just getting to 10-6 and then bounced out of the playoffs you might be able to do that down 3 good starters (or a future Hall of Fame defensive player with some change left over, as the case may be) to those teams with cheap QBs. That equation is prohibitive, however, if the aim is the ultimate prize.