Didn't Levitt and Dubner establish that a standard seatbelt would be safe enough to class as a car seat for children over 2 if they weren't explicitly banned by law.
https://pricetheory.uchicago.edu/levitt/Papers/doyle-levitt%...
And here’s a pretty entertaining TED talk he gave:
I recall when I was young (before car seat laws), the car had shorter seat belts in the back. Almost as though they expected children to sit there . . .