It doesn’t stop working at all. It stops getting free OS updates, presumably because there are many dozens of hardware configuration to manage, and having fallen into that trap once with Android devices, they didn’t want to immediately jump into the same pit a second time.

FWIW, a common thing to do with a chromebook that has gone out of update service is to install either a non-Google ChromeOS like Cloudready, or just install Linux directly. Both approaches are usually easy and effective.

I don't agree that something should stop working of course as that is just a straw man...rather from a pragmatic view of actual usage from my own perspective that seems to be fairly normal to use something else within 6 long years that renders the tech of the now obsolete in terms of attention over gathering dust so thus I stop caring whether it is supported or not.