I agree, there has to be a decision soon [and I think that's probably already been made, maybe by both sides], but "what will the market be at next year's trade deadline?" isn't the thing I'd worry about. Every deadline is different, and with teams getting $4M+ of cap space for next season that will open up a little breathing room for everyone, even if most everyone rushes out and spends it ASAP.
The bigger risk is him underperforming this season - which, while a 32-32-64 pace, still underperforms his pace from the prior 2 seasons - and/or him getting injured. If he's healthy and having some 35-50-85 career year, teams will still pay out. If he's "only" 30-30-60, I think someone will still pay out because his game is different from Guentzel's it's more "he has grit, sandpaper, and can score, we need that in the playoffs." If he's going 20-25-45 and/or gets hurt, teams are going to reel in the offer prices and the Blues will do good to get even what Guentzel got.
That said, I'll be really surprised if he's still with the Blues after July 1.