It would instantly melt billions of tons of snow and send a mud tsunami (”lahar”) straight down the river valley toward Tacoma.
Yes, and one day it will happen. We can do some planning but permanently evacuating Tacoma isn’t going to happen. Human life is short and geological time is long. The odds are that Rainier will give adequate warning. But, maybe not. Life is a risk.
Mt. Rainier is ranked as one of the most dangerous volcanoes in the world. And it doesn’t even need to have an explosive event. (I’m not even sure Rainier is possible to have an explosive event.)
It does have a lot of hot water inside, and a very unstable rock. A small earthquake, landslide, etc. could cause a release of that hot water, instantly melting the snow and ice, mixing with the mud and the lahar.
I have done work where new subdivisions are going in west of Mount Rainier. All of the old trees from the last lahar make putting in sewer lines difficult. I wonder how many folks that buy those homes know they are sitting on 30 feet of old mud and trees deposited just 500 years ago (which reached Tacoma and the Puget Sound)?