Earlier quoted context omitted.
I'm genuinely curious: why do you want to create new directories at the root level? Is this something you're just used to doing and it breaks your customary way of working? Is it a "general principle" thing (which I suspect is a lot of the objection the HN crowd really has at the root, no pun intended)? I've been using OS X-based Macs since 2002 and used Linux and FreeBSD for a decade before that, and I don't recall…
I put all developer libraries that I do not want to put into a system path into /DevelopLibs. On Windows I put them in C:\DeveloperLibs. That way I can build supporting libraries of different versions software under there, eg. /DeveloperLibs/abc-1, /DeveloperLibs/abc-2 and test that I have a self-contained executable that runs on my development system without relying on anything in $PATH. It was a habit on macOS sinc…
There are certainly many decisions Apple makes that I don't like and I am always keeping my eye on Linux in case I really feel the need to pull the rip cord. But for me, this is just not a hill of sufficient size to die on. "Apple now mounts the system partition as read-only" just doesn't feel like a slippery slope that leads inexorably to... well, I'm not really sure where people imagine it's going. "This is something I used to be able to do and now I can't and soon that means I won't be able to open a shell!" I mean, maybe, but... probably not. If I had to choose whether "Macs in five years will only be able to install programs from the Mac App Store" or "iPads in five years will be able to install programs from places other than the iOS App Store" is more likely, I'd honestly go with the latter.
> I don't understand why people would ask "why would you want to do that?"
I guess I don't understand why asking the question is controversial; I'm not trying to lead anyone into a rhetorical trap that forces them to admit only axe murderers would ever use this functionality. :) It's simply that in what I'm pretty sure is over a quarter-century of Unix use on my part I don't recall ever putting anything in the root directory of any of my systems except by mistake.
As for "defending the ability to only resize windows from the bottom right corner until a few versions ago," your memory of this is different from mine, I suppose. I recall a lot of Mac users thinking that Apple's one-corner-only approach was a bad leftover from the pre-OS X days that needed to change!