I don't think Lovie sees Miles as a risk. I think he sees Miles as someone that will work with him day and night to develop the best strategies and watch the film. Plus, Miles has a long tenure learning Lovie's system. As much as people complain about the money, I don't think several hundred thousand dollars for a year is sufficient to warrant sacrificing 5 million + in salary for multiple years.
Whether or not Miles is good is besides the point (we don't know). Lovie knows that he won't have a job if he doesn't win more games. And, he hired Miles. I think that speaks volumes about his decision. Probably his decision as a dad.
As a dad, I know what it is like to believe in your kids. This could be blind fatherly optimism. It could be that Miles is somewhat like an echo chamber, after all, he has learned most of what he knows about football from his dad. But I don't disagree with his decision. Whether or not it works is yet to be determined. And, I would fire Lovie if he doesn't win more games this year.