If Carr has to miss any games due to injury, I think Kubiak will give Hill a lot more snaps at QB to take pressure off of either Haener or Rattler. Hill at QB in a bootleg & rollout based offense would put a lot of pressure on the defense. Making the defense prepare for Hill as a run pass option QB and moving all across the alignment when either Haener or Rattler is at QB would put a lot of pressure on defenses and take a lot of pressure off of the two young and inexperienced QB's.
Neither Haener or Rattler has taken a single snap against a defense full of starters that have game planned and running complex and confusing schemes. We won't know how Haener or Rattler would do against that. Preseason is a mirage when it comes to starting against another team's best starters throwing everything they can at you. That's why a lot of QB's look great in preseason, but struggle a lot during the regular season.
The only QB on the roster behind Carr who has had success as a QB during the regular season is Hill and I think Kubiak knows and values that.