Again, as I said before, saying something rare isn't analogous with saying it never happens. I would point out there that going back 5 years to find an example is proof of this.
I'll admit my scope is a little narrow, because it's rare you can't always trade for a cost controlled "proven" mlb starter. There isn't too many "Spencer Striders" out there with teams willingly looking to deal them.
You have to either draft and develop them on your own, or trade for prospects who are still young and develop them, guys like the ones you pointed out along with Gallen in your previous post. And look, people remember the guys who worked out, no one remembers the Allen Websters or Ruby De La Rosas of the world; there are a lot more of those guys. I think you're including these guys into the young cost-controlled arms, I'm just adding "proven" into my definition, which seems relevant if you're a fan base that expects to compete now. Young, PROVEN, cost controlled, available arms are a very rare commodity.
For the Sox to do such things requires them to trade MLB talent. If you want prospect packages like that, we'd be trading guys like Juran Duran, or Tanner Houck, maybe you can throw guys like Nick Pivetta at the deadline in there as well.
I'm not opposed to trading for guys and developing them, our problem has been two-fold over the last decade or so. Not investing in pitchers in the draft, and not going a great job of developing pitchers, I've been a huge advocate of pouring money into the scouting and development department the last few years. Something the Sox ALLEGEDLY started under Bloom and have continued to ramp up under Breslow. For what it's worth, it looks like it's starting to pay off. Hopefully this is the tip of the iceberg.