I still have my 2014 machine. I've upgraded it with an M.2 drive and more RAM. Everything else is perfectly fine and I wouldn't see the difference with a newer machine. I'll keep it for a long as I can because the longer I wait the better the machine I replace it with will be.
Also I just wouldn't know what to do with it after. I can't bring myself to throwing away a perfectly good machine, but keeping it would be hoarding.