> How do you install Android or some other OS competitor on an iPhone, or install iOS on a Samsung or Google device?

You don’t, you either choose to buy an Android or an iPhone.

This is like asking why I can’t go to Princeton but have my favorite Stanford professor come teach my class, or why I can’t play CS:GO inside Mario Party.

Life is full of tradeoffs. You can’t have your cake and eat it too all the time.

> This is like asking why I can’t go to Princeton but have my favorite Stanford professor come teach my class

You can do that. You just convince them to teach the class at Princeton. Professors move from one school to another all the time. You can also go take a class at one school while attending another if you want to do that, and the other school will not only not stand in your way, you'll generally get transfer credit.

> or why I can’t play CS:GO inside Mario Party.

These are two different games. The example you're looking for is that you can't play Mario Party on a PlayStation. But that's barely even an analogy, it's just the same situation -- why shouldn't you be able to play Mario Party on a PlayStation except for nefarious anticompetitive BS?

This is distinct from technical capability. If you can't play Mario Party on a Sega Genesis because it only has 64KB of RAM, too bad. But if you can't play it on <whatever device> even though you paid for it solely because of some anti-competitive lock-in even if the hardware is capable and you're willing to e.g. create a compatibility layer, that's something else.

Apple owns what they create. Why do you feel entitled to demand things from them?

They sold what they created to someone else. Now the someone else owns it, but Apple still wants to control it.

You either get the money or you keep the stuff. You don't get to take the money and retain ownership control of the stuff.

You didn't buy a full license to their software. You bought a phone that is a complete package. What entitles you to try to install their software on stuff they don't want it on or didn't build it to supprt?

Do you want them to spend money developing iOS for other devices, just so you can take that for free and install it on an Android?

You knew the rules before you pulled out your wallet.

For iMessage - they run a service that costs money. Why should they just give it out for free? Sure, they give it to their customers for free, but why does that mean you get it too?

It would be like sneaking into a complimentary hotel breakfast. Sure, you managed to grab someone's keycard - you technically can get past the restrictions. Still eating food someone else paid for. Just because you have friends eating already and you want to join them doesn't make it right.