As I posted previously more than once (not sure which thread), my take is significantly different than yours.
Several years ago, Toyota started to specify synthetic oil for almost all their engines in order to maintain their warranty. In the past, the have had sludge problems with many of their engine designs when conventional was used and if people did not perform regular oil changes (as many are prone not to do). Given the number of cars they sell every year, and even if only 5% don't change their oil on a regular basis, that would result in a few hundred thousand people per year with sludge, which was turning into a public relations and legal nightmare. You can Google "Toyota sludge" for more info about this.
However, when Toyota started to specify synthetic, just about every competitive brand salesperson used the high cost of Toyota maintenance against them in selling situations, so it was a marketing nightmare. So Toyota changed the requirement and now they don't say you must have synthetic, but they say you must have 0W-20, knowing full well that for 0W-20 there are only synthetic motor oils available that can meet that viscosity spec. Before that, they specified 5W-20 for CAFE reasons, but you can get a conventional oil with 5W-20 rating, so that doesn't fully explain the current 0W-20 requirement.