The fact is St. Louis have been hurt by their trades of 1st round picks over the past 7 years. 11, 13, 15, 18, 19/20 1sts got traded before the draft. 16 1st player was traded away. They got some 1st back in trading statsny and Shattenkirk.
Depending on which pick ends up with Buffalo, the Blues will have made 9 1st round picks between 2010 and 2019/2020. That's hardly gutting the future. These are the players from the picks they traded away since 2010:
Duncan Siemens (2011 1st dealt in the Erik Johnson trade)
Emile Poirier (2013 1st for Jay Bouwmeester)
Jack Roslovic (2015 1st for Ryan Miller)
Joel Farabee (2018 1st for Brayden Schenn)
These are the players from the 1st round picks they've acquired since 2010:
Vladimir Tarasenko (2010 1st for David Rundblad)
Klim Kostin (2017 1st for Ryan Reaves)
Dominik Bokk (2018 1st for Paul Stastny)
You can add Morgan Frost to the top list, but that was a pick the Blues got for the Shattenkirk rental and then flipped as part of the Schenn deal, so it was house money to some extent.
Anyway, the point is that nobody on that top list of picks traded away solves the Blues problems right now. Siemens and Poirier have never amounted to anything. Roslovic is a respectable young player, but not the sort to make any major impact for the Blues right now. Farabee and Frost are good prospects, but so are Kostin and Bokk.
What has
definitely been detrimental is that the Blues have been consistently competitive for a long time, and thus they've basically only had late 1sts to work with. As you said, late 1sts are not nearly as likely to succeed as early 1sts. And what it largely comes down is that most of the best teams in the league right now struggled heavily during the last decade and got some premium picks as a result. Winnipeg has made 5 Top 10 picks since 2010. Toronto... 4. Calgary... 3. Buffalo... 6. The Avs... 5. Tampa... 3. Edmonton... 8. The Preds are a bit of an anomaly, but they hit on their sole Top 5 pick and turned it into their 1C.
The last time the Blues had a Top 10 pick was 2008 with Pietrangelo. Their two highest picks since 2010 are Schwartz and Tarasenko at 14 and 16 (and both in 2010, actually). What hurts the Blues right now the most isn't that they traded away picks. It's that they've been one of the best teams for a long time and the league is designed to make that difficult to sustain. It's the same thing that has happened with the Hawks, Kings, Rangers, Pens, etc.