The way I figure it, the 21 inch performance tires will last 15,000 miles average. If you are very concerned about high tire replacement costs, think of it this way. How much more would it be to have a "beater car", gas it up, carry the insurance for it AND do the maintenance for it? Replacing the tires for the Model S is cheaper still (1,200- not bad; the tires for my 2002 Mercury Cougar were the in the same ballpark AND that car was a garbage- 8 years and replacing the tires every other year).
When I get Model S, I'm keeping my Toyota for the first year, as a back-up see how I make out with the Model S, if the Model S works for snow, great, if not, I keep the Toyota as a winter car/shopping center car/gardening car (to take the dents and abuse that I would not want to subject the Model S to- Honestly that's all a Toyota or any ICE car is good for- as the whipping boy for the Model S).