From a Montreal perspective...
Best case scenario:
Edmundson plays well next year, gets the attention of Seattle who pick him over the eligible young D's (a 3-year commitment can be attractive to a budding franchise), and Montreal only paid a 5th + used some available cap in 20-21.
Disappointing/realistic scenario:
Edmundson predictably tops out as a semi-reliable bottom pair D, Seattle pick someone else (imo Kulak/Mete/Juulsen/Fleury from 7-3-1). His contract's second year might force a necessary choice between the UFAs-to-be (provided one or more of Gallagher/Danault/Tatar/Petry aren't traded by then) but the cap space is still healthy. Once the defensive prospects move up the ladder (Romanov, Norlinder, Struble all "safely" project to be superior) Edmundson has 1 or 2 years left, and should still be the type of low-expectation player for which teams pay a late round pick at the deadline, to beef up their D (if you need all the space you can get for the post-ELCs). Nothing problematic.
Worst case scenario:
Replaces Alzner in the Laval Rocket Museum/Catacombs. Honestly I don't see it as very likely.