Costing $35m in cash next year is a feature, not a bug.
Corbin is a more talented pitcher with significantly better underlying stuff than Hill, who should cost absolutely nothing to acquire, & should be fully subsidized for this season to help them stay under the threshold this year.
& if he actually turns it around enough to be a quality part of the rotation next year? Amazing. $23.3m against the threshold in a year you plan to be a taxpayer anyway is literally just John Henry's money, & even more so with the overage that isn't part of the CBT calculation.
Getting predictably poor pitching from a guy who pitches worse as the season goes on & giving up something for that privilege is not something that interests me.
Digging around, I’m not really seeing anything encouraging about Corbin’s stuff. In the pitch modeling metrics, his Stuff+ is at 80, which is 105th of 109 starters. He’s always been more or less a 2-pitch starter. His sinker/slider thing still seems useful against lefties but it doesn’t seem especially worth it to me. Possible he can ramp it up playing meaningful games, but I don’t think I’d trust him with bulk innings which is what we’d need.
As you said yourself, they’re likely to make some off-market moves none of us can foresee. (Not to diminish the fun of trade speculation).
One thing Bloom and co. really seem attuned to is pitchers with extreme release points. Specifically, lefties with long horizontal release points (Joely, Diekman, Jacques, Bernardino, Bleier, Sheriff are all in the upper decile here) and righties with long release extensions (Whitlock, Jansen, Brasier, Pivetta, Kelly, Winckowski, Schreiber).
Certainly not the only criteria, but there's enough data to show a trend. As a team, Sox lefties have the longest, most extreme release point extension in all MLB. As a team, Sox right-handed relievers rank second in extension point and fifth overall (Whitlock ranks #1 in all of MLB starters). I'm not sure how predictive all that is, and of course it's not uniformly true among all our pitchers (Crawford especially, with his funky delivery, gets well below-average extension).
Of course, this is a league-wide thing; the Sox are just at the forefront.
This SI piece found that one foot of release extension is the equivalent of an extra 1.7 mph of velocity, in the perception of the batter.
All of which to say I think they'll get a guy who either already has an above-average extension point, or can be coached into having a lengthier one (e.g. someone who is pretty tall and "underperforming" their extension). So among name guys, Giolito, Flaherty and Lynn seem like candidates (though to be fair, Corbin is also high up there in this metric). Among the less heralded, maybe Michael Lorenzen or Brady Singer?