Actually it doesn't INSTANTLY turn it into a pile of junk. Even your own post says security updates go back 2 operating system versions, so you've got a couple of years before it's a 'pile of junk'...and even then your 2014 machine will not be USELESS, just not supported with security updates anymore, and eventually browser standards will move on so that you take the risk yourself in using a less secure/outdated browser, and developers decide to make use of some new hardware/API that your machine doesn't support.Actually it does. Security updates only go back 2 systems. So on 2023 my mac mini 2014 will be useless. And if big sur is the last update then it is useless. Browsers will only support for so long. And others apps too. Even the high sierra my 2014 mini came with already feels dated.
I have a G5 IMAC sitting in my basement boxed, Useless now. No browsers or email or other main apps work. There is 104FOX. But all sites are not supported. And when they stop that the its mo good.
The last G5 iMacs were produced in 2006, so complaining that a 14 year old machine is useless (or not supported anymore) is just...WOW!!!
My 2008 octo-core MacPro is working perfectly well on El Capitan (10.11.6) despite not having any security updates since 2018. Safari/Firefox/Opera still works, as does MS Office, Adobe CS6 (which feels faster than the latest Creative Cloud on my newer machines). Certainly not useless by any means.