100 years? Really?
How many 100 year old cars so we see on the roads today? A handful - most are in museums or only rolled out for vintage car festivals.
You could argue that there were less cars back then. Fair enough, but how many even fifty year old cars are on the roads? Even pre-2000 cars are rare, and easily identifiable due to the older format registration plate.
I think in 100 years nobody (or very few people - less than ten) is going to be regularly driving a Ford Zetec to the shops. They’ll be in museums or driven on special occasions or to vintage car rallies.
if we just take this on face value for a second, its perfectly possible people will still be able to use petrol vehicles as long as parts are available and petrol is available. This is the same as using anything after its no longer on sale. Its also possible that petrol engines will be converted to use hydrogen fuels or a new blend of petrol engine fuel, but the mistake being made here is nobody really has any need to as theres nothing intrinsic in needing petrol engines for anything. If it was like oxygen or something maybe it would last forever but its basically old technology thats been replaced, technology which moves people and goods from A to B with greater efficiency. In this case the Horse was replaced, which replaced just carrying things around in bags.