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