I ended up just going out and buying a bunch of old ThinkPads and running OpenBSD and Linux on them. I think I got like five computers for less than the price of a single MacBook Air.
I'm pretty much done with proprietary operating systems because they clearly no longer provide the best experience, from my perspective.
I'm tired of being locked out of software I paid for because I "upgraded" the OS (looking at you, Logic Pro), or vice versa, unable to use new software because I didn't "upgrade" (most new Mac software won't run on my MBP). If I upgrade, I literally throw away a $250 piece of software and have to spend another $250 (or whatever) to use the new version of _that same software_. At least open-source OSes allow me to stay up to date while still having nearly-maximum agency over how the computer works and what software it runs.
I'm tired of my computer doing a ton of stuff I didn't tell it to do. The amount of notifications/nags/etc. on a modern Mac/Windows computer is absolutely mind-boggling to me. The always-online aspect, software-update-checks, "telemetry" and "analytics", phoning home every time I launch a program...
I'm tired of my computer not being a tool for me to use, but rather just a gateway to a bunch of stores and ways I can spend money, with a bit of "productivity" sprinkled on the side. I could go on and on.
Oh, I didn't even touch on the software design/UX aspects. Like Riccardo wrote, it's a worthy of an entire article itself. I could probably spend an entire day documenting/critiquing the degradation of user experience in Mac OS and Windows. Mac OS had a lot further to fall, and indeed has been extra-disappointing because I used to hold it in such high regard :\