This is the PK Subban trade of goalies.
If he's good then you just got a really good deal out of a player who fits a need on your roster and helps you contend ASAP. Granted the PK deal was trying to help decide whether we should sign Taylor Hall long term as well.
If he sucks, you cut bait and try again in a year or two. Fitz is probably closer to get firing though.
The alternative is if you trade for Ullmark/Saros with more pieces and give them those huge contracts and they implode, well, you just nuked your entire contending window in one swoop.