To be fair, you can always pull a Sapporo or Moscow and run the trains outside, but covered.Or 1987, if you don't count that modest extension - which wasn't on the island!
The completely underground rubber-tired metros are perfect for Montreal weather conditions. When I used to live there, I don't remember ever having service stop because of weather issues.
But where it doesn't work is in the less dense suburban towns and boroughs - and particularly off the island. When Drapeau announced Line 3 in the early 1960s (essentially the Central-Deux Montagnes branch of the REM) it was steel-wheeled.
If there's ever another line that runs east-west through downtown Montreal, using the existing technology makes the most sense to me.