I think it should have a built in kill switch. It knows it's own EOL date, and just wouldn't work after that.
Really? I know that would be easier for developers, but think about it from a "it's my computer" point of view - do you really want people doing that to you? It's your computer, you should be able to to run whatever software you like. A reminder to upgrade, OK, that's fine. But actually disabling? You go too far.
If you want this kind of choice, the only way is free and open source software. That's the reality, and it's been that way for a very long time, just read the EULA. It isn't yours. They're not doing things "to you" by preventing it from working, they're protecting themselves. Free software could decline to work by default, with something like an about:config opt out feature. That's because free software belongs to everyone.
Next, hardware. Apple, the most among all the companies making hardware, explicitly reserves the right to modify the hardware after sale. There is in effect a hardware EULA. Your rights are fairly limited to turning it off or disposing of the phone. You don't have a right to modify it, you don't have a right to jail break it, you don't have a right to run software that Apple doesn't, through its license that you've agreed to, approve of. Not only is the software not yours, there is tacit residual hardware right reserved by Apple. I don't like it, you don't have to like it, that's how the agreement effectively works though. So if you really don't like it, then you shouldn't buy those kinds of products.
Android, it's fairly similar license wise, but there is at least in most implementations the option to enable 3rd party software installation. So you could install some ancient unpatched web browser if that's what you wanted to do. And of course Cyanogenmod and other free OS's you have a lot more control because the software is yours by virtue of you having possession of it. You have a legal right to modify it if you don't like how it behaves.