We have people here that bike year round, even though our temperatures dip well below 0C (we get -25 to -30C on a regular basis, with windchill being -35C to -40C). That is far more effort than I'm willing to put in.
Once it snows, I consider it far too dangerous (though again, there are a few people here that bike in snow w/ studded tires). I only have a bike lane for a short part of my ride, the rest being on 50, 60, and 80kmh roads with no paved shoulder. I don't want to find out what happens if I hit a deep patch of snow, ice, or some cross-winds while a semi is passing me doing 80kmh.
As for your 2nd point, I made that mistake the first time it got close to freezing. By the time I made it to work, I had a tough time breathing.
I'm in Edmonton, which sounds pretty similar to your climate. Except for those few -40 days I've found winter biking to be fairly comfortable once you get moving.
In my experience you don't want to be using a bike lane after heavy snow anyhow; they often don't get plowed. If you can take residential roads you're pretty safe (and studded tires really do make a HUGE difference).
But yeah, you don't want to be on the road with cars going 80km/h - I wouldn't be comfortable doing that every day in the summer, either.