People just do not understand physics and MOFT. Yes, under most circumstances a 20-grade oil has sufficient HT/HS to prevent excessive wear. That is why the grade is "back spec'd" to older models. Plus automakers did get retroactive CAFE credits.Way back in early 2000's I had a Ford 4.6l V8 - and they back spec'd from 5w-30 to 5w-20 via a technical service bulleting. That was obviously not due to cafe, so anyone remember why they did so?
There has been all kinds of speculation here over the years I have found in old posts - from Cafe - which can't be true since Cafe has never been back dated - to Ford wants to kill your old car.
However the physics of wear are not some black art that nobody can understand. Even you can. Wear increases with decreasing MOFT, and it is around a typical 20-grade that the curve increases most sharply without design changes to an engine. Since I keep my automobiles for a significant period (and I wish to provide headroom for fuel dilution) there is zero practical downside to an oil with a higher HT/HS. The reverse is true as well, that there are no benefits to an oil with a lower HT/HS besides a marginal fuel economy improvement.