The idea was that Bloom could have identified players that would be good / bad major leaguers and made a call on players and moved them while they had trade value.
Bloom could very well be a good (or very good) GM, but plenty of other good / very good GMs make deals before other teams get wise. Sure, the Sale contract was moronic and got him out of town, but Bloom's immediate predecessor has made an entire very successful career doing this. Cherington did a pretty decent job of this while he was here (the only player that seems even remotely to have come back and bitten him was Reddick as part of the Bailey trade). Theo did a good job of this too (the only prospect trade I can truly think of him really "losing" was Rizzo for Gonzalez, and even there Gonzalez was a good player, but not the replacement to Manny Ramirez some were hoping for - as a complement to Ortiz - not as a RHH OF, obviously) many were hoping for.
Aren't there two different things here? The first is saying that Bloom should have identified prospects who wouldn't have developed and cashed in - and the second is saying his predecessors did a good job of not trading valuable prospects without getting a justifiable return. On the latter, Bloom hasn't deal a prospect of consequence yet from what I've seen (maybe I'm missing someone)
Looking at Dombrowski, and we agree entirely he did a great job trading who he traded and keeping who he kept in terms of those choices (with one exception that's hard to mark him down for), these are the Top 10 prospects he inherited going into the 2016 pre-season with their "grade" from Minor League Ball as well as other notable names (for instance Dubon and Marrero were in Fangraphs Top 10 and Ball had a 1st round pedigree)
Yoan Moncada (A)
Rafael Devers (A-)
Andrew Benintendi (A-)
Anderson Espinosa (B+)
Michael Kopech (B)
Sam Travis (B/B-)
Brian Johnson (B-/B)
Wendell Rijo (B-/C+)
Luis Alexander Basabe (B-/C+)
Michael Chavis (B-/C+)
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Deven Marrero (C+)
Mauricio Dubon (C+)
Trey Ball (C+)
Of the "Big 4" he traded two and kept two, and it's hard to argue he made the wrong call there. Devers is a star and Benny was a plus value OF until his power got sapped. Moncada has shown flashes but is a mixed bag and Espinosa got sidelined by injuries. Kopech went in the Sale deal and while it'd be nice to have a Kopech...it was much nicer to have a Sale and win a title. But going down the list, he left a lot of those guys in the system and didn't convert them into anything to help the club and they eventually petered out. The best MLB player on this list outside of the Top 5 is Mauricio Dubon, who Dave Dombrowski did trade for Tyler Thornburg (this happens, no shade on DD) but it really goes to the point that every GM will have prospects who just flame out and they don't convert to pro value (the second best player is probably not on this list, Santiago Espinal, who the Sox traded for Steve Pearce and who has been a valuable utility cog in Toronto...but we all know what Pearce did in 2018). Prospects are hard, assessing if they'll be pro contributors and converting them into something better is even harder. Even a great front office that often wins trades and is generally considered S-Tier does this. Here's Tampa's Top 10 prospects from 2020
Wander Franco
Brendan McKay
Vidal Brujan
Xavier Edwards
Shane Baz
Brent Honeywell
Shane McLanahan
Joe Ryan
Josh Lowe
Greg Jones
They still have Brendan McKay who hasn't played in the MLB since 2019, Vidal Brujan who has been a nothingburger at the MLB level, and Greg Jones (who is doing...okay at AA/AAA). The three Top 10 guys they don't have any more (Joe Ryan, Brent Honeywell, and Xavier Edwards) are all MLB contributors of varying quality (Ryan is good, Honeywell and Edwards aren't as good). They kept the best players - Wander (well...), Baz (if healthy), McClanahan, and Lowe, but it's a tough game to properly assess that and other teams are gonna have their views too.
I think what is fair for people doubtful of Bloom is how he picks the prospects he'll send out for that major contributing talent - we just don't know that yet as he hasn't had to do it or done it. Tampa historically survived by being sellers and accumulating cost controlled talent to use in future years, and in Boston he only had one buying deadline and did well with Schwarber but that's very different than what we think). But I can't pin a failure to deal Bryan Mata (BTW, the #3 prospect he inherited...jeez that farm was a disaster) on him and more than I can pin a failure to deal Brian Johnson on DD or Henry Owens on Cherrington for instance. Bloom's ultimate fate in Boston will likely depend on which of these prospects Bloom does trade for MLB talent (or what MLB talent he trades to make room for these prospects) and how that plays out but if we're going to be mad at Chaim Bloom for not converting Bryan Mata or Brandon Walter into something else (especially when we are pining for any pitching talent we can get), we'll never be happy.