It's analogous to buying flood insurance. It's super expensive because the only people who buy it are those who live on floodplains.
If the only people who bought car insurance are the riskiest drivers who are clinging to less safe forms of transport, and the rest of the pool that used to contribute premiums are now in safer, cheaper autonomous vehicles and different forms of insurance, then the manual group would need higher premiums.
Unless future drivers become more accident-prone than today's drivers, their premiums will be similar. I don't see much incentive for adverse selection.
The point GP was making is that as autonomous vehicles replace manual, the likelihood of an inter-vehicular accident should also fall for the remaining manual vehicles, lowering their premiums. A nice positive externality - more reason to subsidize autonomous vehicles.