At which point it’s not getting that mpg.
Plus if I had a dollar for every time I had more than 2 people in my car, I’d struggle to buy a Big Mac meal. Empty space for your shit is what rear seats are 98% of the time when you are single.
When the time comes you hop in somebody elses car who has the seats.
With your logic minivans are allowed shitty mpg because they can seat 7, even if they rarely do.