Thing is, say you took Randy Johnson, and at 24 said, "Hey kid, you're going to close." In single-inning intervals, could he have been as dominant? I'm not going to argue he would be, but I think it's possible. deGrom just had an ERA+ of 216 in basically three years' worth of innings for a closer; could he repeat his performance if he were pitching an inning at a time? That's probably what impresses me most about Rivera--it wasn't just that he was dominant as a closer, but he was that dominant for so many years.
I still don't like comparing relievers to starters, though. Not using any stat. Though it's the same physical activity, the requirements of the role, to be great in the role, are very different.