Just five years ago, a price-conscious auto shopper in the United States could choose from among a dozen new small cars selling for under $20,000. Now, there’s just one: The Mitsubishi Mirage. And even the Mirage appears headed for the scrap yard. At a time when Americans increasingly want pricey SUVs and trucks rather than small cars, the Mirage remains the lone new vehicle whose average sale price is under 20 grand — a figure that once marked a kind of unofficial threshold of affordability. With prices — new and used — having soared since the pandemic, $20,000 is no...