The doubt comes in because people feel that argument becomes circular: fewer spots leads to greater separation leads to less competitive semis (or hypothetical quarters) leads to argument for fewer spots
I don't accept that the number of playoff spots has any connection to the distribution of quality players and coaches. The reason a handful of teams have appeared in the playoff every year is because the playoff is only a few years old, and programs move in multi-year cycles.
For example, in the BCS/CFP era, Ohio State has had two really good coaches (Tressell and Meyer) both of whom have stayed for years (9 years and 7 years, respectively), and neither of whom were fired for not winning enough. But before Tressel was hired, Ohio St finished unranked for three straight years, and in the year between Tressel and Meyer (when Luke Fickell was HC), Ohio St finished unranked. There's nothing magical about the name "Ohio St" – yes, they have fans, history, resources, etc.... but it still takes a great coach to keep them at the top. We'll see if Ryan Day is Tom Osborne to Urban Meyer's Bob Devaney, or if he's Larry Coker to Meyer's Butch Davis.