Technology

Tesla has much higher output than the number of vehicles that spontaneously combust. So the supply shortage you envision won't occur unless cars light up by the hundreds or thousands a day. And that's before accounting for all the extra slack in production created by decreasing demand around the world.

Teslas combusting damages the brand. It's a material risk of owning a Tesla. It would also mean higher insurance. In fact beyond some combustion threshold, Teslas would become uninsurable. Therefore Teslas on fire directly contributes to decreasing Elon's purchase power.

In fairness, I think the possibility that someone might light your car on fire is a stronger incentive to not buy a brand than internet posts making fun of the brand. It also means now they have to beef up security at all of the dealerships. Car sales are surprisingly impulse driven, hence the famous high pressure sales tactics, so it'll be harder to get people in the door if they're strip searching everyone and if they can't have as much inventory on the lot, customers can't drive home with the options they want so less impulse buying. Also full coverage insurance, required for financing, will get more expensive or even impossible to obtain which possibly means a massively decreased customer pool.

I think a better counter argument is the environmental damage caused by lighting all the cancer boxes on fire.