You seem to be assuming that all motorists calculate their costs down to the last penny. Most motorists I know have decided that whatever the cost may be, they want to own a car. They'll hunt for a bargain, but once they've bought the car and secured a second bargain on the insurance, they don't fret about saving every penny on every journey.
No, I don't think that motorists calculate their costs to the last penny.
I think that motorists never calculate the total cost of their cars and just see them as a series is small costs, each of which they pay without thinking about the total amount that they pay. However when they think about rail travel and they see an annual cost of a season ticket at £1,500 they baulk at the amount and say "I can drive it for less".
Even though for many, epically those that pay more than £500 a year in insurance and £1,000 a year in purchase costs then they would, even with some fairly high milage driving, could be better off not having a car.
As an example those that lease a car for £120 a month would be paying £1,440 a year.
Alternatively, if you buy a £12,000 car and sell it 3 years later for £7,000 (bearing in mind most vehicles depreciate at about 50% every 3 years) is £1,667 a year.
If you have finance to purchase the car than that would push your costs up higher. In the above example, let's assume that you sold your last car for £7,000 and you have saved £1,000, with an interest rate of 5% on the remaining £4,000 you would pay a further £100 a year in interest of paid back over three years, circa £120 a month in repayments
Therefore, it could easily be that you could be paying:
£1,800 a year in purchase costs (+£800 on my previous numbers)
£800 in insurance (+£300 on my previous numbers)
Which would push the total costs to £3,000 per year before you drive anywhere. Therefore to get to a point where you're paying 35p per mile then you'd need to travel 12,000 miles.
However, even 35p a mile is still quite expensive for mileage costs. As an example Basingstoke to Guildford by car is 25 miles (11,300 miles a year to and from work). A season ticket on the trains is £3,672. Which gives you a mileage cost of 32.5p per mile.
2.5p a mile doesn't sound like much difference however to get your driving costs down to that you would need to do a further 2,000 miles.
That's on a rail journey where it's further to travel by rail than car and where it's also possible to save a further £200 by having a Woking to Reading season ticket (which is valid Reading to Basingstoke and then to Woking and valid Reading to Guildford and then to Woking, such means, although technically not what it's designed for you have a valid ticket when using services between Basingstoke and Woking and Woking and Guildford).
Yes, after that you would then only be paying fuel costs, however most people live and work fairly small distances (i.e. Less than 25 miles apart) and the smaller the distance you travel the higher your cost per mile rate is and the more likely it is that you'd be better off not owning a car. Especially if you have to regularly pay for car parking (just £3 a week would add a further £150 to your motoring costs a year, or a further 650 miles a year to keep your milage costs down at 35p a mile).
The really crazy thing to do is to own a car to drive less than 5 miles each way to a station, and pay for parking, to then travel to London by train.
If you can use buses then often the season tickets are significantly cheaper, especially in urban areas, where chances are you could get a bus season ticket for the town you live in for less than your insurance, failing that most of the rest could achieve it for less than their insurance and servicing costs.