2 million miles? perhaps 200k is correct?
I have a Forester and didn’t switch to snow tires. Managed to get around OK, even in the year when we got over 100 inches of snow. But I’m a conservative driver and I could avoid driving in the worse conditions.
But if you want maximum performance and safety, switch to 4 snow tires.
Note that a FWD car with 4 good snow tires performs better in the snow than an AWD car with all-season tires. (that’s without considering the effect of studded tires). If you are used to driving with 4 studded snow tires, you will find that the Subaru has much less ability in snow and ice, and is much less safe.
Also note that you must keep the tread depth (as well as tire model) equal on all 4 wheels or you will cause expensive damage to the subaru.