Re: Chapman as a starter?
Starters can get into routine - guaranteed to pitch every 4-5 days. Relievers may pitch 5 days a week, or not pitch at all for a week. The latter would definitely be more stressful.
While I have no way to tell how many innings he can handle as a starter over a season, I would tend to believe that he could handle more than 70-plus, easily. Comparing the total number of innings for a starter and a reliever is apples to oranges, I think.