I have no problem letting the gathered prospects develop, I just have a problem with waiting for them as "plan A".
Almost all the teams that are considered playoff contenders at this moment aggressively went out to find players, or at least tried to, that can both help them now and in the future. They drafted well, sure, but they didn't sit there and let the world go by as they did. Boston was serious in trying to revamp their defense from day one. They collected three first rounders so as to trade up and get Noah Hanifin. The very next year they aggressively sought after Shattenkirk, and had Loui Eriksson wanted to sign here, he would be a Bruin. The Penguins traded their top prospect, at the time, to land Phil Kessel in hopes of providing more scoring depth. The Lightning were adamant about shoring up their depth last offseason and went to sign plenty of free agents like Girardi to do so. Then they traded Brett Howden, Libor Hajek, and their former first rounder Namestnikov to really shore up their defense. Then there are the Predators, who honestly built their forwards up through trades and signings as opposed to drafts. I mean, three of their top-6 forwards were acquired through trades (Ryan Johansen, Kyle Turris, and Filip Forsberg). The only real example I can think of teams just waiting patiently was the Winnipeg Jets, but they had to collect a lot of top-15 picks to do so. All of the other teams took a stance to draft well, yet also search for other external means for help.
Hell, even the Blues aren't abiding by just waiting. They were linked with Drouin at a time last year. They have been linked to Hoffman and Pacioretty at various points this season. It's been speculated ed nauseum that they wanted to use the Winnipeg 1st at the trade deadline. The Blues have done a great job accumulating assets through the draft, but those assets become useless if they don't help the team in some way, shape, or form. The Blues have the likes of Robert Thomas to look forward to, but what happens to Zach Sanford? He has to beat out Nolan Stevens, Erik Foley, Ivan Barbeshev, Adam Musil, Tage Thompson, and Sammy Blais to snag a bottom-9 spot. That's just trying to beat other prospects. Nevermind beating out other roster players, who all have much more experience, as well. (p.s. I wasn't trying to pick on Sanford in particular, just using him as an example)
The point I'm trying to make is that the Blues have a prospect pool 15 guys deep that can all reasonably make the NHL in their lifetimes. There are almost enough forward prospects to start a whole other team with. That's fantastic, don't get me wrong. But there is a point where the marginal benefit of keeping one more guy decreases to nill. I think the Blues are pretty close to that point honestly. The only real way the pool becomes better is by adding better talent to it, but that becomes increasingly difficult unless the Blues are drafting top-10. Adding more of the same isn't going to help. So if a talent that can help us now and in the future comes along in a trade, I don't see a reason to look away because the prospect pool can become 18 players deep instead of 16-17.
For the record, I'm not trying to suggest trading everything for the sake of making headlines. I'm not even saying that you in particular are suggesting the Blues avoid hypothetical trades that would have Tyler Seguin come to the Blues for Blais, Stevens, and month's supply of jalapeno poppers or something. I just think that there is such a thing as being too patient as well as too agressive, and I tried expressing it in the most laconic way possible... something I failed at by the look of this post.