Coaches start young and rookie QBs all the time when they could be starting better veteran QBs. They do it because at some point young QB needs to play to get better. Most of them are going to struggle for at least a year, but they could end up being better in the long run than those currently better veterans and that coach could then win more games in the long run.
In our situation, a new HC is very likely to at least get 3 years to prove himself before he would get fired. He's got two years to build the team in the draft and find a young QB before the cap is clear and there will be real expectations of winning anything.