You are going to need to grandfather users of existing used cars if you don't want a mob with pitchforks and torches. Even when electric cars are affordable on average its necessary to understand the effect on price and value that would be caused by changing incentives. There are around 250 million fossil fuel cars on the road and $160/gallon fuel taxes would make those cars value effectively go to zero while sending electric cars price to the moon.

Even if scaled in over time to avoid breaking the economy you would have a long tail of cheap used cars which would be difficult for users to up and replace owned by those least likely to be able to absorb the shock.

You have folks commuting in a $5000 car for 10 years for which $20,000 would be a financial death sentence. There are plenty of folks who absolutely need a car to be able to work and continually keep an older used car for a LOT less than average.

Not all of these folks could even get a car loan for a compliant car so you would be basically telling them to go fuck themselves.

It makes more sense to simply mandate phasing out manufacturing or importing new gas powered consumer cars and let cars age out rather than using extreme gas taxes as a lever.