I won't deny that there are many many users that do not care. However, I disagree that there is a real choice in the current duopoly. Android allows you to install applications outside the app store. However, you trade this possibility for continuous data harvesting by Google and only a limited window of updates.

So, the choice is largely:

some freedom ^ (privacy | long-term updates)

Freedom, privacy, and long-term updates are all important to me, so there is no real choice.

Because of antitrust concerns. https://www.investopedia.com/terms/a/antitrust.asp

Because morality does not revolve around the question "Will a megacorp lose some revenue?" There are many things companies and individuals could do that would make them money which they are nonetheless constrained from doing because those things are wrong.

Also, the idea that users understand these issues and are passing judgment on them when they buy an iPhone doesn't seem realistic for the majority of iPhone owners. If you walked around polling iPhone users about this issue anywhere besides Silicon Valley, the most common answer would probably be "What?" And that's ignoring the fact that someone might object to this but still buy an iPhone for unrelated reasons. So even if their dollars could act as moral validation, I don't think it's reasonable to say they do in practice.

Competitive markets ultimately benefit all users even if users aren't smart enough to promote their existence by voting with their wallets.

Kind of like having fire exits benefits users even if people don't pick retailers based on their existence.