You can't get an OnlyFans app in Google or Apple's app stores either, much as I'm sure the company in question would like the exposure of having one, because much of that website's user generated content also falls foul of Google and Apple's content policies. I don't understand what appears to be a commonly held view that it was fine for the appstore duopoly to deem content unsuitable for their store until the content in question was calls to hang the vice president and shitposts about Jews.

Yeah, I think that both of those things should probably have more public input when the players at hand effectively control the entire market for smartphones in the US.

We have strong bill of rights restrictions on governmental power because of the massive amount of power held by the government. So having that power be controlled by small numbers of entirely unaccountable figures with more power than the government in certain areas doesn't seem better at all?

I think the main thing is if you want to make a case for there being anti-trust or user freedom or dubious moderation priorities issues with app stores, a campaign centred around examples other than Parler is much more likely to win widespread support.

Regulatory capture prevents competition. Money influencing politics prevents regulation.

What regulation prohibits you from making a phone OS, even for an existing phone? Or your own android build? Or your own store on Android?

Up to a certain point. No amount of money would prevent congress from toughening bank regulations after the financial crisis. I also doubt that the republicans are going to give big tech companies a pass when they come back to power.

Market share is low, but there certainly are other mobile OS options out there. The fact that people in general don't want to use them doesn't make Android & iOS a "duopoly".