I agree, but I don’t see regulators enacting laws to allow that competition to emerge.
That is very difficult for smartphone ecosystems, because there is is really hard to reach the critical mass. Even Microsoft started its own OS for smartphone, but you can't force app developers to create apps for an OS that is only used by a few people.
Sometimes those big companies can't buy there rivals, but they can still buy out their employees. Microsoft did that with Netscape for example. A convenient way to get rid of a rival. Of course you can't force people at Netscape to continue working on the browser.
There seems to be a big difference in the thinking of the US and the EU. The EU is more focused on the consumer, while the US focuses on the companies. Food safety is an example. In the US companies can add new ingredients to food unless they are proven harmful. In the EU it is the other way around. First a company has to prove that something is NOT harmful. Then it can be added to food. As a result fried bought in Europe bought at McDonald's have hardly any of those extra ingredients, while in the US fries contain a lot of stuff most people can't even pronounce.
Medicine is another example. The US have that giant opioid crisis because so much harmful stuff is available in the US without the need of a subscription. It seems profits of the pharmaceutical companies are more important than saving people from addiction. In Europe we even limit how much can be charged for certain pharmaceutics. So while one does of chemo therapy costs $46,000 in the US, it is just a tiny fraction of that in Europe.
Tizen and KaiOS are available as well. People can still choose. However, realistically, there is a reason there are only 2 'major' OS choices and it comes down to manufacturers being too lazy to develop their own. Android is an effective monopoly for non Apple manufacturers. And it’s a choice these manufacturers openly make. They reap the benefits of stealing/using your data. Side loading is not the answer for 2 operating systems, it’s having a choice of operating systems.
Sideloading actually gives consumers more choice, because they now have the choice to sideload or to continue using only Apple's App Store. At the same time the EU restricts how much data companies can collect. So in the end the consumer might have the choice between two good options instead of having to choose the lesser of two evils.
We also needed the EU to make sure that you can keep your phone number, if you switch your phone provider. Of course you can still get a new number if you want, but it is much more convenient that you can keep your own. That strengthens competition. Not sure how the laws are in the US.