Trying to respond to everyone at once...
Quoting: Shibbal18
How does StL afford 8.65 mill being added to their already cramped cap situation
So as many have pointed out, St. Louis is going to have significant cap space opening up, with three defensemen hitting free agency. Meanwhile, no significant forward is hitting FA for them. Since Bowmeester probably is not going to be resigned, Edmundson shouldn't expect much of a raise (probably not above $4.5), and Gunnarsson can probably be brought back for cheaper, the 8.65 if worth it for them uniquely, specifically because of how David Backes fits into their team immediately. Meanwhile, St. Louis gets the starter they've needed for years.
Quoting: ON3M4N
Last 15 Games
Halak - 2.99 GAA / .899 SV%
Rask - 2.22 GAA / .924 SV%
But hey lets trade Rask and let Halak be our stater
This is true and I can't deny this. However, Rask is one of the few pieces that this team can likely move to make cap space for McAvoy, Carlo, Heinen, etc. Because he has put up good numbers lately, his value is still high, enough for the Blues to take a buyout-proofed Backes contract back, while still getting a pick back. I don't love to do this, but the only other alternative I see is trading Krecji, and I think that's a tougher move to replace. There are quality goaltenders on the FA market this year.
Quoting: yet_another_armchair_gm
Ignoring the trade, Erik Karlsson is in no way the FA the Bruins need.
I made the assumption here Zdeno Chara retires. If that happens and the Bruins have the cap space, I think they would be well served to get Karlsson. That being said, I also considered Panarin/Stone in the discussion also.