If we presume the 'fair share' for Model ≡ cars is a similar percentage... And the profitability of the cars starts at 12% for the base version... Then we are talking $4,200 profit on a $35,000 car... And a whopping huge $473 of that profit margin earmarked to support Superchargers per car. Since there will eventually be ten times as many of the Model ≡ being manufactured as Model S, that means that anywhere from 1,500-to-2,500 cars per Supercharger... Thus, providing $709,735-to-$1,182,892 more revenue for each Supercharger... Not bad when one considers each location of 8 stalls costs in the neighborhood of 400,000 to install (per Pungoteague_Dave).
See that's where I think the math actually starts to break down. There are currently somewhere between 60,000 and 70,000 Tesla Model S and Model X cars on the road in the United States. Let's assume it's 60,000 for the purpose of the math. As of today, there are a total of 260 supercharger stations open with a total of 1,707 charging bays (data from
supercharge.info). So currently there are only 230 Teslas on the road per supercharger station, or one charging bay per 35 cars. In many areas, these superchargers are nowhere near capacity, most of the time, but we have seen lines at supercharger stations (particularly in California) at busy times like holiday weekends.
The cost of building those stations, according to your $400K per station math (which I haven't verified, but seems reasonable) would have been around $104 Million. If you take that $104,000,000 and divide the number of cars sold in the US, that means it has cost Tesla roughly $1700 per Tesla Model S or Model X sold to build out that supercharger network. So if costs stay constant on the build-out, and they want to maintain the same ratio of superchargers to cars on the road that they have today, that would mean they'd need to collect $1700 per car sold to build out the SC network at the same pace. And this doesn't take into consideration the electricity and maintenance costs associated with keeping the SuperChargers up and running. If we take that into consideration, the amount required per car may be something like $2,000 (or possibly higher).
Now at this point, all we have are rough numbers and estimates of things. Tesla would have a better idea, based on actual monitoring of Supercharger usage, as to where they need to build out more supercharger stations in order to satisfy growing demand. They'd have a better idea (at least better estimates) as to what the required vehicle to supercharger ratio is, too. But estimates are as good a starting point as anything, and I think the above math is sound, based on current numbers.
The bottom line is can Tesla afford to "give away" an option that may cost them $1700-$2000 per car in a car that is expected to sell for only $35,000 (half the current entry level Model S)? It's already an aggressive goal to build a Tesla and sell it for that price. And I'm not sure they could afford to also include lifetime SuperCharger access in that price and still be profitable. But really it depends on how far they can drop the other costs associated with the car. And that's something only Tesla knows (or will figure out over the next 18 months).
I just don't think anyone should think that Tesla broke a promise or let them down by not including lifetime SuperCharger access in a car at this price point. It hasn't been promised yet. It *may* not even be revealed on March 31. Or if it is revealed, they may not reveal the final cost of the option until closer to production.
With the Model X, the only details we had on pricing at or near its reveal were that it would be a "slight premium over the Model S." We didn't know the actual pricing of options and base models until November of 2015, about 6 weeks *after* Founders cars started being delivered and after Signature deposit holders had access to the design studio. I expect we may be in for something similar with the Model 3. Details of battery pack options and pricing will most likely not be revealed until late next year.
