Ultimately, you’d sell bugger all cars on the basis of “Guaranteed for 20 years / 500,000 miles” or at double or treble the price of a standard car. I don’t think it would be a major technical challenge – I’ve been in plenty of Merc Taxis that have done massive mileages and seemed to be fine. Your biggest problem would probably be the interior wearing out.
The majority of new car buyers are interested in having a new car – status, image, technology, increased fuel economy, lower noise levels etc. They’re not really that interested in real long term longevity. So who the hell would you sell them to? Tight arses like me are happy to motor about in cheap second hand cars from reliable makers…
Or to take an example from another sector – I’ve got a Nokia 3310 and an Ericsson R310 knocking about the house somewhere. They’re both reliable, better at making calls than my current smartphone and have longer battery life. Bugger all chance of me using one as a day to day phone though…