You still need to throw fastballs here and there to keep them honest (see: Bello 2024). If you develop pitchers that are unable to throw fastballs, they will struggle in MLB IMO.
The Sox best development arm right now is Dobbins. He throws is 4 seamer (not a cutter, not a splitter) 40% of the time!
Tolle is the Sox most exciting starting prospect in the lower levels, because of his fastball shape!