Ars mentions that Apple (on average) now supports new Mac’s for 7 years, but even though Apple stops delivering updates at least the (non-Safari) browsers and other software may continue to receive updates for quite a bit longer.
In this day and age browser security is the first and most important line of defense, and as long as your browser is updated and your firewall is up you can have some sense of security.
I personally never touched a Chromebook, and have no idea how hard it is to get Linux onto them, but it sure proves Stallmans old argument about freedom.