Youre trading or best forward prospect, our best RD prospect and arguably our best LD prospect, along with two firsts. That's how it destroys the future.
What is the future of the Blues organization? If you're trading for Tavares the future is now. You're not trying to win a cup 5 years from now. You're trying to win the cup now.
Tage Thompson might be the best forward prospect but I'd argue that overall Kyrou will be a better player and he'd still be a Blue.
Schmaltz is possibly the best RHD prospect the Blues have but at best he will play 3rd pairing minutes behind Parayko & Pietrangelo. Walman is a decent prospect but his ceiling is probably top 4 LHD. You still have Dunn for that position and possibly higher.
Schwartz is a top line winger that the Blues currently are overstocked on. Steen, Fabbri, & Schwartz could all play top 6 LW and probably all could play on a top line. It's a trade from strength.
Maybe you get away with giving up 1 1st instead of 2? I'd do it either way.
Here's the roster:
Fabbri Tavares Tarasenko
Steen Stastny Perron
Berglund Barbashev Paajarvi
Sanford Brodziak Reaves
Bouwmeester Pietrangelo
Dunn Parayko
Edmundson Bortuzzo
That gives you a total team salary for 2017-18 of $58,239,167.
If the Cap remains at 73 million you are left with $14,760,833. The Blues have to sign Parayko and Paajarvi.
Let's say Paajarvi gets something like a $1,750,000 - 2m/year deal and Parayko gets a 7 yr 5,850,000 deal. Together they're in the neighborhood of 8 million signed which leaves you with 6+ in cap.
Let's say Tavares signs for 8.5/yr. That would start in the 2018-19 season as next year he's still only making 5.5.
That 2018-19 season you would have a cap starting around $53,345,000. That season you would have to resign Fabbri & Stastny. Fabbri would get a hefty raise but Stastny probably takes somewhat of a pay cut. Edmundson, Brodziak, Reaves, & Perron would either have to be replaced or signed. Those guys aren't core players. Let's say Fabbri goes crazy playing with Tavares and Tarasenko and he gets signed to $5.5 and Stastny also signs for 5.5. That puts the Cap up to $63,345,000 with 4th liners and a 3rd pairing D to replace.
Again you're left with plenty of cap space.
I guess I need more information on what your definition of the Blues' future is. My definition of the Blues future, and I think the organization thinks similar, is a win now mode. If the aforementioned deal was done it puts the Blues in a good situation to be contenders for the next 5 years.