However, by that argument, we should just impose a high tax on all combustion engine cars, or perhaps simply introduce an appropriate carbon tax, instead of banning the combustion engine.
Some of the reasons we don't want to do that are:
1. A carbon tax is regressive and might result in the middle class still being just about able to afford cars, but the lower class completely unable to, which would be unfair.
2. By forcing the shift away from the combustion engine with a ban, rather than just nudging away from it with a tax, we're providing a stronger spur to innovation.
Both of these reasons apply to supercars to a large extent:
1. Elites being able to buy combustion engine (super)cars, while the rest of society cannot, would be unfair.
2. Forcing companies to innovate in the case of supercars, which provide a greater engineering challenge, increases the chance that the all the relevant engineering obstacles are solved for "normal" cars.
IMO the two end-points:
1. We just introduce a sufficiently high carbon tax, together with appropriate redistribution to make up for its regressiveness, and let the free market decide whether combustion engines are still affordable after internalising the externality.
2. We ban the combustion engine on the basis that we're all in this together and we want to make sure that the transition definitely does occur.
are defensible. The middle-ground where we tax supercars, but ban all other combustion engine cars is far less so.