> There is no reason why Apple should have to support any of that.
No one's asking them to support it, but there's a large difference between not supporting and actively thwarting.
> How exactly? There was and is nothing to stop a competitor bringing out an alternate platform
The answer is in your own phrasing. You talk about platforms. Apple has so restricted their products that the only way to compete is to build a whole platform. You can't build an app store, there's nothing to run it on. You can't build hardware to compete with Apple, iOS won't run on it, you can't replace the OS because the hardware requires signed code. The only place to compete without bringing a whole platform is Android. If Android didn't exist, there would be zero question as to whether Apple is a monopoly based on their actions, and I don't think that the fact that there's one other provider necessarily absolves them of the responsibilities for their actions. The anticompetitive behavior underneath is the same regardless.
> The thing is what Apple, and Google did is very, very hard. It takes massive investment and very sophisticated technology.
Not nearly as hard as you make out. The market is so lucrative that it makes sense for investors to throw some billions at a capable competitor and eke out some percentage of the market, but they don't. They don't because everyone is so locked into their ecosystems that any competition is at a real disadvantage. That's why the behavior is anticompetitive. It prevents competitors from entering the market to provide choice. A duopoly is obviously not enough if there's no competition at all in some areas.
> It's just rational customers acting in their own interests to prefer one product over another.
Is it? What if a separate app store existed that guaranteed that if the app existed on multiple platforms, your purchase was good for all platforms? How many more people might feel free to switch platforms and try new things if they knew they could get most or even all their purchases moved with them?
I generally buy Samsung phones, ranging from top of the line to budget models over time. There's a Samsung app store. I've never once been inclined to use it when the Google play store exists and means if I get a non Samsung phone that my purchases are still good. If there was an app store that worked on iOS and Android, I'd likely us that instead. That would be one more barrier preventing me from getting an iPhone at some point if I thought it was worth the switch. The same goes the other direction.