Its not just performance, its injuries. The farther away a prospect is from MLB, the likelier it is that he will suffer a major injury before he gets there...and HS pitchers are behind college kids from the get go.
Its not only HS pitchers, its all young pitchers....but the HS kids have the highest risk factors. Let colleges carry the risk for the first several years out of HS.
Anyway, look at the top pitching prospects of 2018. Nine of them in fangraphs top 30...five of them were 60 FV.
All but one were at least Espinos age. All but one were just as advanced and had success in the minors to that point. Most were more advanced than Espino.
Whitley
Honeywell
Alex Reyes
Sanchez
Kopech
Mitch Keller
Kyle Wright
Buehler
Puk
One...Walker Buehler...is what everyone expects Espino to become, an immediate sensation.
For the sake of discussion, how many teams would have done quite well by trading any of them...when they were in AA...for 3.5 years of Sean Murphy?
All but one, the Dodgers.
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There were 23 pitchers taken in the first round of the 2016 draft. You would expect a first rounder to make MLB six years after being drafted. You would expect a first rounder to have some early impact. Thats why you draft them and pay them big bucks.
Out of those 23, nine were HS kids.
Five have gotten to MLB. One, Ian Anderson, has more than 1.1 fWAR...3.9.
Fourteen were college kids. Thirteen have made MLB. Three of them have been converted to relief. Four of the rest have at least 2.1 fWAR.
Eric Lauer....5.1
Quantrill...4.3
Dunning...3.2
Dakota Hudson...2.1
Tell me again why anybody would draft a HS pitcher in the first round.