Agreed. It takes more than a few developers to support older operating systems.

At my old job we supported only two versions of our software product, Tanzu Operations Manager versions 2.10.x and 3.0.y), and we cut new patch releases every few weeks (similar to Apple's cadence). Bumping dependencies was a pain. Well, usually it went fine, but sometimes you'd hit a gnarly incompatibility and you'd either pin a Ruby package to a known version or try to modify the code just enough to make it work without making a major change.

If I had to put a number to it, I'd say it cost us 2 developers to keep our older product line consistently patched, and our product was a modest Ruby app, much less complicated than an entire OS.

You act as if we should be thankful for 6 years of support when the hardware and sane support cycles easily exceed 10 years. And those aren't 6 years of security updates; they are 6 years of forced yearly feature upgrades and breaking things along the way.

my favorite Intel MacBook is from 2015

Not on Macbook Airs that are only 3-5 years old though. We have a number that we plan on replacing after EOY, but we are still using for now. Can't get Sequoia.