The question is not whether Travis Shaw could reasonably have been expected to become an All-Star, the question is whether Travis Shaw could reasonably have been expected to play ok defense and put up a .310/.420 line at the plate. Because that's more valuable than all but the most elite relievers. This was a bad trade because it was an everyday position player for a middle reliever with a minimal record of success and a questionable elbow history. What happened after the ink dried has no bearing on that assessment.
How many .730 OPS position players can you play at 3B in a given game and get that value out of them?
A .730 OPS is basically Brock Holt, and despite 3B being one of Holt's weakest positions (at least by my visual assessment) he's still substantially better there than Shaw had been in his Red Sox career.
Pablo Sandoval was unmovable sunk cost with a realistic potential of giving a roughly .730 OPS with defense likely in the ballpark of what Shaw had provided to date.
Marco Hernandez also looked like a reasonable bet to give a roughly similar OPS, and also with better defense than Shaw.
Even Josh Rutledge looked like a roughly analogous piece to fill the 3B hole.
Meanwhile in a world prior to Joe Kelly - elite setup man and Matt Barnes/Heath Hembree mid-relief dynamic duo acquiring Tyler Thornburg appeared to offer a very valuable upgrade to a bullpen that was one man deep (Kimbrel). I don't think anyone would disagree that Shaw represented more theoretical raw value than Thornburg, even based on his weak overall 2016 performance, but that was seen as unusable value given how fungible the 3B mix for Boston was, while Thornburg v. his alternative appeared at the time to be a pretty meaningful and usable difference in value.
I'm not one to go to bat for this or any past (or probably future) Sox FO when they trade position players and/or prospects for relief help. I didn't like the Gagne trade. I didn't like the Bailey or Melancon trades at all. I didn't like the Kelly/Craig deal and I'm including that because I said the day it was made the only value we'd ever see from it was when they finally moved Kelly to the bullpen. I hated when they revisited the trade the Brandon Lyon/Suppan deal to include Freddy Sanchez. I was ambivalent towards the Melancon for Hanrahan trade because it was our broken reliever for your not (yet) broken but more expensive reliever, and the Carson Smith for Wade Miley deal worked because after watching Miley for a whole season it was pretty damn clear the horseshoe was just about to drop out of his ass any day now. I didn't like the Kimbrel trade either, though acknowledged then and now that it's a hell of a lot smarter to overpay for a workhorse elite reliever with a long track record than someone with a smaller sample and dubious health.
But I don't think anyone in the Red Sox FOs for any of those deals would claim they thought they were getting even 1:1 total value. They were all made by a team believing it was gearing up for a title run and/or filling a far bigger hole and as a result willing to overpay to make it happen. They were all bad deals in terms of total value but where viewed (almost always incorrectly) as a slight overpay to get a need piece. I have a hard time thinking any FO would move away from that rationalization. Look at what the Cubs paid for a rental on Chapman last year. A veritable king's ransom but they finally won a WS and Chapman was a big part of it. When it works you're a genius, no matter the price. When it fails you're roasted for years to come. That's the life of every decision maker for every MLB franchise.