If you chose to use iAnything then it's a bit late to start complaining about lock in now.
However, sure, lots of users chose Apple knowing exactly what it is. Apple's not going to change since their model clearly appeals to lots of people.
If you don't like Apple's model, then don't choose Apple devices. What everyone else chooses is somewhat irrelevant to you. (Other than network effects noted earlier.)
I can hack up a "device" with a raspberry pi zero or whatever and call it "HaxyDeck" and claim it is all open to anyone who wants to tinker with it, but at the end it'd be irrelevant because only me (and perhaps a couple other people) would have it. The aspects you want to ignore (number of users, be something other than Apple, what others are using) would actually affect my use of HaxyDeck directly: since i'd be the only one (or one among a tiny number) using it, i'd be the only one having to make it do things i want, it wont have software from others, it wont support software other people may want to use for communication, software some services that theoretically have nothing to do with phones or computers (e.g. banks) wont work because HaxyDeck's userbase is irrelevant for them, etc. All of these have to do exactly with what others are doing.
Basically see how all the non-Android Linux phones (like PinePhone) are faring. You can't just ignore what effect having a large user base some platform (be it a device, an OS or even service) has and say "just use something different".
They are hardly irrelevant, especially if you like money.