Assuming they can maintain that level of supply as demand drops. Some the economies of scale they operate with will be lost as demand slows and then the logistics that requires large and ongoing investment may slow considerably, although much of the latter is probably outside the 20 year horizon.

It could play out very differently on the consumer side though. If electric vehicles became 50% (or some arbitrary significant percentage) of the market then a lot of local gas stations will close, the ones that remain may have to start charging higher prices (again losing the logistics and scale advantages of today). There may be some tipping point where the whole retail side of the industry finds itself in a death spiral where gas stations are charging more and in less locations, making oil powered cars less convenient and driving more people to electric vehicles.