Alternatives are a lot easier along the 401, particularly east of Toronto, with alternate routes (although weather can have broad impact) and multiple VIA trains. The reality is northern Ontario is not the 401. You may wish that services were evenly distributed across the province but they're not; they're reflective of population density and distribution. A few million people get more services than a few thousand. No one wakes up in Marathon intending to drive to the Soo only to find the highway closed then expects a train to be waiting for them. You want Toronto-level service? Live in Toronto. You paid roughly 1/4 to 1/5 for a house there than in the Toronto.
The real impact of major highway closures is their impact on the movement of goods, and passenger rail isn't going to solve that. For giggles, compare the traffic volumes of the 401 vs 11 or 17, particularly in the winter. Dig deeper and compare the volumes by vehicle type.