I have two laptops in addition to my workstation, so I'm thinking that probably contributes to why I've never felt the need to keep around parts for another system. I used to keep around an extra GPU (first a GTX 560, then a GTX 760) for an eGPU setup to use with one of my laptops several years ago for a gaming HTPC. After I got a newer laptop, I got rid of the extra GPU as well. Now the newest of my laptops has an RTX 2060, so I'm clearly done with eGPUs too.
I can't even claim to use the desktop computer I built for my parents as a parts dumping ground anymore because their build is newer than mine. I put together a Skylake build for them five years ago with a Core i3-6100 on an ASRock Z170A-X1/3.1 motherboard. The only hand-me-downs in that machine now are my old SeaSonic S12-380 380W PSU from something like 12-15 years ago and one of my old 500 GB SATA hard drives as an external backup drive.
I do have a box of old-ass cables that need to be sorted through though. A few months ago, my neighbor asked me if I had a spare VGA cable he could borrow for a few days. I did, and for all I care, he can keep it. I think there's a 15' USB 2.0 A-B cable, a few CAT5 or 5a cables, and a couple of older HDMI 1.x cables in there as well, but probably nothing that uses obsolete connectors that can't work with newer hardware at the older revision's throughput. However, I know I won't be using any of it again, so I'll have to go through those soon and take another haul to the electronics recycler.