Re the YYZ experience, the airport authority's take is that a complete shutdown due to snow and icing, such as LHR has done, is not a viable option. They have worked very hard and invested large sums to prevent it from happening. Yes things slow down significantly when the going gets tough, but many flights still get through.
I've had the pleasure of seeing their snow clearing team in action up close and it's pretty impressive. They run a conga line of equipment and clear the centre 150 ft of a runway with one pass each way along the runway. The whole thing takes about 10 to 15 minutes.