Manufacturing, generation of electricity with heavy use of fossil fuels (could be changed and is changing in many places, luckily) and particulate matter (for example through tire wear) mainly.
Then they should get much smaller and lighter. If they have to get their tires and brake pad replaced for wear every so often, where are those particles going? Unless they evolve tires and breaks that don't shed particles in the water and the air, or get very very light batteries.
Yeah. Surely buying more tech stuff will save us. We just need to sell even more cars, but "ecological", and the car pollution problem will be solved in time, with even more cars, but less polluting this time! Truly a marvel of engineering.
And with those batteries, they're heavy as shit, doing increased damage to roads and bridges, and are outright disallowed on smaller rural / suburban bridges.
There's plenty of issues with making a fully enclosed vehicle sustainable regardless of drive train. Just the amount of metal needed of any kind.
They will probably always be needed for last mile deliveries or people with certain disabilities. That said, if we could get most North American cities to just 20% bike usage for people's major commuting choice, that'd be transformative.