I think most of us would prefer to build from the farm up and not pay the ridiculous contracts. In fact, many years ago Bill James said that the way to use free agency is to fill spots that aren't filled from within and that trying to build an all-star team through free agents won't work. Even the Yankees dynasty (and i do hate to use that word in reference to the Y's) was built around their "Core four". However, the Sox have a history of not being able to recognize/develop starting pitchers but a solid history of bringing along position players.
IMO prospects are good for two things - to develop and bring to the majors or to use as trading chips - in the position the Sox are in, probably for pitchers. Therefore what we're doing is trying to recognize and keep our best position players and trade the rest for pitching. Even Moncada, as much as I hated to see him go, was expendable because 1) we didn't have a need for him with all of our OF positions filed, and 2) because he's still a prospect. Given the failure rate of prospects I'll trade a blue-chip prospect for a proven top-flight pitcher every time.
It's easy to say in hindsight that we would have tried to work out a package for Sale or Quintana, but at the time the Sox made the Kimbrel trade the Pale Sox had a solid pitching staff and we had no reason to think they'd part with Quintana or Sale. In fact, IMO, if Sale hadn't made himself personna non grata in Chicago he'd still be there with Quintana. So we did what the FO felt they had to do. (Had it been up to me I'd have tried to milk at least one more year out of Koji, but that's another story).
My biggest gripe about what happened in this offseason is that we didn't pursue Ziegler harder. I know there was chatter that he was going back West...but we all know how that worked out. Ugh.