Oh yeah they could easily keep runs open well into June, but unfortunately for us, Grouse makes about 70% of it's profit from the summer season. Since March, the vast majority of people going up Grouse are pass-holders, not ticket buyers.
I may be wrong about Grouse's winter insurance ending the May long weekend, but I'm fairly certain they'll do what they can to get their summer preparations underway. With that being said, 6 meters of snow is 6 meters of snow, and I wouldn't want to have to move all of it.