A fantastic point!
Unfortuneately undercut by absolute truth (rather than per vehicle comparisons) that
* the number of vehicles (both in the US and globally) has substantially increased in number since the 1970s,
* many of the older vehicles with the poor performance are still active on roads somewhere,
* the nature of vehicle use in western countries has changed - while the efficiences for both cars and SUVs may have improved, the proprtion of SUV usage (with lesser efficiencies) has significantly increased.
I wouldn't advocate killing all humans and animals, but there's much to be said for halving the number of high consumption humans ...
I'm comfortable getting by for months on end sans most things - global field geophysics was an extended jolly.
That kind of progress has questionable impact on net emissions. https://en.m.wikipedia.org/wiki/Jevons_paradox
If you wanted to be extreme in CO2 reduction, I guess your best bet is to kill every living human and animal on the planet (to be clear, not advocating this!) Clearly that's "too high" a price to pay.
On the other hand, you can be very practical and look for CO2 reduction in ways that don't harm people's current lives. Increasing vehicle mileage is an example of this - mileage has doubled and emissions per vehicles halved since 1975.[1] In fact, I suspect this is going to get even better very quickly. My Highlander Hybrid, which is a large vehicle, gets roughly 50% more MPG than in this chart.
So we can say we're already doing a lot (and that's just one thing.) The idea that somehow the line between acceptable and unacceptable just happens to be right between suburban and city living feels a little too convenient for me.
[1] https://www.epa.gov/automotive-trends/highlights-automotive-...