nothing new about relievers being failed starters- the best closer of all time was a failed starter
As are almost all relievers. Many just fail earlier than the MLB level. People are enamored of the Yankees bullpen, for example:
Chapman - Started in Cuba. Started in the minors as well (badly) and converted to a reliever in AAA.
Betances - Started all the way up to AAA, then had a year in which he ran up a 6+ ERA there. Reliever pretty much since.
Ottavino - Started all the way up MLB with the Cardinals. Wasn't good (4.60 ERA in 3 AAA seasons). Converted to relief by the Rockies.
Britton - Started for several years with the Orioles. Was awful. Immediately lights-out as a reliever.
Green - Started up through AAA (and was actually good at it there) but got 8 starts in MLB, ran up a 5.94 as a starter and then was converted.
Also, reliever performance is so fickle from year to year. For example, I don't think Matt Grace is any good, last year's numbers notwithstanding. I'd be surprised to see him put up a sub-4 ERA (by the way, he's also a failed starter). Ask the Yankees about Tommy Kahnle (never a starter as a professional, by the way).