Swihart was also drafted by Theo. If Swihart's lack of development is the fault of Cherington, then shouldn't the development of Betts, Bogaerts, Bradley, Shaw, Benintendi, Moncada, Kopech, Devers, Vazquez, etc. be to his credit to the same degree? Or do we only hold GMs accountable for their failures?
Cherington only lead 4 drafts for the Sox. While he did draft a few ancillary relievers (Johnson, Poyner, Maddox and Buttrey), he did not get much pitching. But he did acquire ERod.
Most of the young ace type pitchers that fans want to see come up through the system have been an issue for Boston because they have been picking later in most drafts. And they haven't really picked early often enough to avoid normal attrition of prospects. Really, they took one shot and it didn't work out.
Whil their drafting of pitchers has not been good, your expectations might be too high. You expected a Roger Clemens? Clemens is one of the greatest pitchers in MLB history. Those guys don't come along every year.