“and the real rule ought to be “whatever you buy, keep it as long as you can.””
yes, indeed, that IS the real rule for big-ticket items, esp. autos ... and sometimes the best buy is indeed a new one, depending on the model and circumstance ... and an extended warranty for the latest hybrids is no joke: those things have a boatload of complex and expensive electric and electronic components, so a single repair on one of those puppies can easily pay for the extended warranty ...
the rule is similar for major appliances, only now, repair is even more essential than new (esp. if one can do their own repairs), because older appliances are often better than anything one can buy new ...
Top load washers and dryers with mechanical timers can be rebuilt almost indefinitely.