When the market value of loss-making Tesla Inc. passed that of Ford Motor Co. this week, it was a perfect illustration of investors’ love affair with disruptive technologies. Investors seem to have forgotten another valuation comparison they should think about more often: In October 2008, the market value of Mattel Inc., maker of Matchbox toy cars, passed heavily indebted Ford.

Old hands in the markets like to say that one year in seven investors worry about balance sheets, rather than profit. It has been eight years since...