I was a little surprised that Rosario came up in late 2017 instead of May 2018, but yeah, he's a 6-plus guy too. It's just that his "plus" isn't as lengthy as that of deGrom or Syndergaard. I don't recall the last time a highly regarded prospect made the Opening Day roster with no prior big league experience. There was Doc Gooden, but I doubt that he was the most recent example. It makes sense to do that delay. Sure, Alonso may be just a dim memory by 2025, but if having someone else hold the position for April of 2019 gives you that extra year of control, why not do it?