i do not believe that apple allows you to sideload apps without jailbreaking (which voids warranty iirc). Therefore, the user is not free to make that choice.
You are free to make that choice [of running software from more than one retailer] by choosing a different computing platform. Hundreds of models of Android phone are available at every price point, from super-budget to super-premium.
It's important to me that I can choose a platform that has a strongly walled ecosystem because I greatly prefer the advantages of it more than I dislike the disadvantages. That's my opinion and I am happy that a product exists that satisfies my needs. If you don't like Apple's ecosystem, don't buy their products.
Asking for an Apple device that uses the Android/AOSP business model is like insisting that Jeep builds a Wrangler with a Tesla drivetrain. If that's really what you want and you're willing to put the effort in, you can make it yourself. It's entirely possible.
Or even if Jeep started adding specific "features" that attempted to make it difficult for people to modify their car.
Apple used to literally sue people for this stuff. That's immoral. And the courts shutdown those frivolous lawsuits from Apple.
It is perfectly reasonable for people to modify products that they own. If you don't like then, then feel free to not modify your product. You own it, and all.
The market has your back though, because Android gives you a very different experience from Apple.
You are free to make that choice.
Meanwhile, I'd appreciate if you didn't object to my right to choose a managed ecosystem if that is what I prefer. I like the Apple ecosystem the way it is, thank you very much.