This is the central conflict here, because only one of two things can be true. Either:
A) iOS is so big that companies can't afford not to target it, and that they're forced to follow Apple's rules, even if they don't like those rules.
or
B) the smartphone market is open, and companies can just decide not to target iOS, in which case there really isn't a way to force Facebook to give you a privacy-respecting app.
If B is true, then having 3rd-party app stores on the iPhone would change almost nothing. If A is true, then it would change everything.
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Often, the pro/anti-Apple arguments boil down to one side saying, "just develop for Android if you don't want Apple's restrictions", and the other side saying, "just don't install a 3rd-party app store if you don't want one."
But both of those responses miss the point. The point is that Apple users want a monopoly. They want to be part of a restricted platform that is so big that Facebook and Google and Microsoft can't afford not to develop for it. Meanwhile, people like me want Apple's walled garden to be small enough that it can be ignored.
In that debate, it doesn't matter where you draw the lines around Apple's walled garden. "No one is forcing you to use iOS" and "no one is forcing you to install an app store" are basically the exact same argument; the only difference is whether they define Apple's walled garden as encompassing the hardware or the software. But the reason why Facebook can't abandon iOS is the same reason why Apple can be considered anti-competitive. It doesn't matter whether Apple's walled-garden is a device or a store, the question is whether or not companies are forced to enter it in order to remain competitive on the market.
On some level, we all know this -- that's why on the anti-Apple side we recognize that "just go to Android" isn't really a good argument. But that's also why the pro-Apple side doesn't accept "just use the official app store" as a solution to their problems. Because what they want is the complete opposite of what software-freedom advocates want. They want developers to be forced to target the walled garden. That's the conflict.
I have a locked down Linux box that only pulls software from a few sources. The difference between my Linux box and an iPhone is not the security, or the privacy, or anything like that -- the difference between my Linux box and an iPhone is that companies like Facebook do not feel obligated to write software for my Linux box.
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And honestly, I don't know that there's a way around that conflict. Either you economically force Facebook to target iOS's walled garden, or you don't. The best middle ground I can think of is something like the Web, an aggressively sandboxed runtime that companies are economically forced to target the same way that they are forced to target iOS, but that is not controlled by a single corporation and that approaches security in a more equitable way. But even on the web, this conflict still exists a little bit. The web being an Open platform means that outside of the platform-level restrictions, I still can't force companies to build their websites/service exactly the way I want them to. At best I can use the Web platform itself to circumvent their services, block ads, etc...
> If you want a system that works they way Android does, use Android and leave me alone.
So what all of this boils down to is: I strongly suspect that if Android development was as economically viable as iOS development, and if it was genuinely possible for large companies to just stop supporting iOS and to "leave iOS users alone", that iOS users would be very unhappy with that outcome. You don't really want to be left alone, you want Facebook, Google, etc to be forced to build apps for you inside of your walled garden.