I pretty much only use computers to create. If I want to consume, I use a phone or something. I don't consume much.
So, like, if I boot up my Air from 2009, it's to write AS3 code and make a demo or something. If I boot up the one from 2015, it's probably because I want to use the Eclipse dev environment I was using then for a specific task. Both those things will simply not run if I upgrade the OS.
And not everything you didn't create is untrusted. I trust certain devs and ensure that if I install something, it is safe.
But it's much more valuable to me to have a snapshot in time of my own dev environments than it would be to have a piece of old hardware running a newer system. The hardware is barely worth anything. The state of the machine as it was, with all the software that was running perfectly on it at the time, is priceless.