I don't have kids, so I'm definitely coming from a place of ignorance. On planes and trains I never see car seats used, which I assume is because crashes are so unlikely on those modes of transit.
If self driving cars are as safe as, or safer than, a trip on a train, would it still be a concern?
Not sure about trains.
I assume that if/when cars are as safe as a plane/train, adults wouldn't even need seat belts but you'd still need a carseat for an infant/toddler because it also stabilizes the child's position. Kids <6mo have no neck control. Kids <10mo can't reliably sit up for long durations. An autodriving car might be safe, but it'd probably still need to make emergency stops / turns. That'd be real bad if there were no carseats.
https://www.faa.gov/passengers/fly_children/
It's a PITA to use due to the huge variety of seat dimensions and car seat sizes and no standardized latching system for plane seats.