That's why the bootloaders of phones should be required to be unlocked by law. You can always root your phone and install a custom ROM on it to keep up to date.

No not really, formula is no more than 10% production costs pa unless w produced poorly to begin with. It’s even less if you’re running multiple versions of roughly the same thing then the costs are spread over those versions.

Yes and no. Installing last version of android on a pixel 4 is most likely absolutely fine. And keeping at least security support is likely not a big deal. 3 years of security update support it is clearly a finance department decision. Why 3, why not 3 and half? Why not 4?

Just because they need predictability in sales, and they attached the support to the "classical" number of years after which you'd like a customer to buy a new tech product. 3 years has always been a magic number for hardware companies, since forever

Because PCs are from a time before tech monopolies. They are based on a spec that allows different vendors to work together. As a result there is more competition, more options for repair, etc.

It kinda is. Windows 11 won't run on older hardware and end of life of the latest version of Win 10 is coming up in 2 years or so. And a bunch of PCs weren't really ready for Win 10 when that replaced Win 7/8 and again, support for those dropped at some point.

Lifetimes are usually more lenient with PCs, but it still happens. You can switch to Linux of course, but then there are alternatives for many smartphones as well.