Well, my guess is:
He is around 55. He loves his job. Retirement isn't for another 10+/-. If a guy loves his job, why quit and why retire. If he still loves the job by normal retirement job, I would expect him to continue on. Could be 70+ years old by time he retires.
Who takes over depends on who is remaining on the bench as assistants. In a Missoulian article about the assistant coaches around a year ago, Cate has stated she is satisfied with the things are now, but she would like to move on to be a head coach somewhere if not here. So, the question here is, Will she wait?
Annette Whitaker gave up looking for head coaching opportunities after she had her children in late 80s and not interested. Patricia Duce may be possible by time Selvig Retires. Could be an outsider.