But I personally would not like having the system I bought as a phone suddenly change in function allowing side-loaded software. (Similar to how I'm not happy that, in time, the PC-controlled Oculus hardware I've owned for several years will soon be useless without an FB account.)
This is exactly the problem. Apple can and does routinely change the functioning of their phones, and if you prefer the old functionality you have no recourse, because Apple contractually forbids you from installing software of your choosing.
I fail to see why people think this is such a problem on this forum. It’s as people think all hell and doom breaks lose if, god forbid, developers are able to distribute apps themselves.
It may be a problem to you, but for other people it's a feature. Some people have more involved threat models than you are keeping in mind. They don't want their device to be able to run arbitrary code or unsigned code. Having this feature but having a toggle to turn it off defeats the purpose.
Maybe you disagree, but HN is shockingly flippant about dismissing this extremely legitimate security concern, and comments like this are a perfect example.
Is that really so difficult for you? Why do you need to prevent me from installing other apps stores, when you can just not do that yourself.