~ Linux should be a hobbyist thing for people who want to spend their time tinkering with their computer, not a way to support a not-very-old device. ~
Whether it comes to phones or computers, I disagree respectively when it comes to custom ROMs or Linux.
So? The problem is both Linux and a Custom ROM take a few orders of magnitude more technical skill and effort to install and maintain, which is completely unreasonable to expect from a typical non-hobbyist retail technology user. Such users should be able to click "update" on their system, get up-to-date with patches, so they can go on to do what really want to do (which probably isn't "maintain their technology"). I'm even someone who's capable of doing that, but I don't want to because I've got much better and more important things do with my time now.
Even if that's true, most computer users don't install Windows. It comes preinstalled.
> It's an excellent way of getting more life out of semi-old systems.
Maybe so, but it's not nearly as good of an option as getting continued support for your preinstalled OS.
Pre-installed with a decent support plan is hard to beat regardless if it's Windows, MacOS or something like Linux Mint.