What do Dellin Betances' historical comparisons tell us about his future?

Dellin Betances is having a historic season so far. After failing as a starter, Betances has been able to harness his electric stuff to become one of the best relievers in baseball. By both RA9-WAR and fWAR (1.2 and 1.3, respectively), Betances has been phenomenal. He has struck out batters at a 15.17 per nine inning clip, and he has allowed only six earned runs, ten walks, 16 hits, and only one home run. If he were to continue at this ridiculous rate, that would put him at about 3.8 WAR over one season, tied for fifth on the single season fWAR list for relievers from 1990 to the present. So, let's suppose that he does pitch that well for the rest of the season (even though he's likely to have bumps in the road). What kind of company does that put him in, and what does that say about his future on the Yankees?

To examine this, here are the best relief seasons since 1990 where the reliever was not the primary closer:

There's a clear outlier in this sample. With Mo kindly placed aside (because we do not expect Betances to be the greatest closer of all time), the other players are still pretty encouraging comparisons. When someone has a season that good, it reveals a couple of things: if a pitcher can pitch that well, they deserve to stay around; and secondly (and most importantly), it probably will not continue. The single-season fWAR clearly dominates the career picture of each player--even Mo had about 10% of his WAR accumulation in that one year. And while they appear to be once-in-a-lifetime performances, each player certainly sticks around for a decent amount of time. Although that does not guarantee Betances will be on the Yankees for that long, it certainly inspires confidence that he can.

If there's one thing that this small sample of modern comparisons shows us, it is to cherish the performance. No reliever is ever able to match such a performance in back-to-back years, so love it while you can. Secondly, they have the ability to stick. Betances, given the comparisons, could very well have a career like Francisco Rodriguez. If I were to make a prediction, I would say he would finish somewhere around there--5-10 years in the big leagues and accumulating approximately 10-15 WAR.

Since Betances was once considered a "failed starting pitching prospect", that would not look too bad. So while it would seem nice to believe that Betances is a perennial 3 WAR player, we have to realize that he will probably never pitch like this again. He will probably not be a Hall of Fame closer, but he's at the beginning of what looks to be a successful career as a reliever. Cherish it now while he's at his best.