Samsung only offers the 7 years of support for their flagship series of phones. The majority of phones they sell are cheaper ones that do not get that support.

This argument would only make any sense if you believed that the reason Americans were buying from Apple and Samsung was for their software update policy, which I'm pretty sure you don't.

I think you could argue people are buying them for longevity, and these measures increase their perceived longevity.

I know a lot of iPhone users who will only buy iPhones because of their longevity. They'll keep them for many generations, and they simply don't have faith in Android phone brands to keep up that long. Phones are large purchases, sometimes financed, like a car. So when it comes to buying, some consumers treat them like cars. They weight reliability and longevity very high, and they'll often buy slightly used phones. A generation or two old iPhone is really still top of the line.

A lot of that is marketing perception but there's also some truth to it.