“More cargo space” is the battle cry of the “bigger is better” crowd, so I reject that argument outright.

Then I reject your consideration of a Kei Truck as cargo space obviously isn't a concern for you.

But, the 1990 Honda Civic was 20 inches shorter and 4 inches narrower. An updated Honda Civic with an equivalent drivetrain as the bolt would be smaller and lighter...

Unrealistic as it could not be sold. A 1990 Honda Civic lacks crumple zones and other safety features that are now required on vehicles in the United States.

...and thus be getting even better mileage than the bolt.

Nope, the additional safety features make the footprint lager and the vehicle heavier.

Those extra 20 inches in length and 4 inches in width are necessary for the bolt to meet modern emissions standards.

Sigh, the Bolt doesn't HAVE any emissions. It's a straight EV. The extra size over a 1990 Honda Civic are for the required safety features and drivetrain.

Yes, the bolt has better economy.

So what's the problem? A domestic auto manufacturer DOES in fact make something that would work and that something is superior in every way to a sub-compact from 30 years ago. Go buy one.