I think the 30% estimation is low.
From 10 years ago, 15 of the top 20 guys made the majors (75%). From 2020, 14 made the majors and that doesn't include Mata and Song who were on 40 man rosters (70%).
If we go by SoxProspects rankings (Fitts granduated) the are already at 15%:
Anthony MLB
Mayer MLB
Arias will be MLB (almost every top 5 prospect has made MLB)
Perales - on 40 man, will be MLB
Tolle - will be MLB
Valera - maybe?
Garcia - on 40 man, will be MLB
Dobbins MLB
Romero - will be MLB
Clarke - will be MLB
Early - will be MLB
Sandlin - will be MLB
Bleis - maybe?
Cespedes - maybe?
Gonzales - maybe?
Paez - maybe?
Soto - maybe?
Monegro - maybe?
Taylor - maybe?
Azocar - maybe?
I'm already at 55% with enough maybes and about half of them will make it and half will fizzle out, which brings us to about 75%.