What about the year before that when the moved Shattenkirk, then the room came together went on a run to make the playoffs and made the 2nd round? You can't use the bad examples and then just ignore the good ones.
Also doing that twice, and the lopsided Reaves trade allowed the Blues to gather add 3 top quality assets to their asset/prospect pool to bring it to the following total: 2016 26th overall(Tage Thompson), 2017 20th overall(Robert Thomas), 2017 27th overall, 2017 31st overall(Klim Kostin), 2018 14th overall, 2018 25th overall(Dominik Bokk), 2019 1st round pick. That's 7 high end assets in 4 years. Allowed them to trade Thompson, 2017 27th overall(Morgan Frost), 2018 14th overall(Joel Farabee), and 2019 1st round pick for Ryan O'Reilly and Brayden Schenn, shoring up their top 2 centers for the foreseeable future all the while maintaining a high end prospect pool with guys like Thomas, Bokk, Kostin, and Kyrou. Sure they're off to a bad start this year, but can you imagine how terrible they would be without Selke caliber defense and 14 points from ROR in 10 games and 9 points from Schenn in 10 games? I imagine it would be quite a bit worse. They're set up for the foreseeable future now thanks to being able to figure out their biggest weakness(Center) and at the same time continue churning out those high level prospects to add to their roster.
Or they could have went for it and held onto those expiring assets or even worse traded their 1sts like Minnesota did in 2017 for Hanzal and lost 1st round, or Boston did in 2018 for Nash and lost 2nd round. I'm sure that combined 1 playoff series win for those 2 teams was much more satisfying than the Blues 1 series win in those same years. I'd say your example is a point in favour of trading expiring assets rather than the point against you seem to be trying to make it.