This is probably a controversial opinion but I actually use my iPhone because it's locked down with a curated app marketplace and secure payment system. I don't want alternative payment methods or app stores. So I find it distasteful that other companies are seeking to control Apple's product design through the legal system. They're essentially trying to make it impossible to purchase a product I want, which is more…
It's not controversial, you can still have your walled garden as-is. The point of this is so that there is the possibility of escaping that walled garden, arguably welcoming more users into the ecosystem. Nothing would change for you. Just like android users can keep using all things Google, they have the possibility of installing apps from other sources.
If my apps are changing, yes it is changing for me.
Right now I can manage all of my app subscriptions from the Subscriptions screen in the Settings app of my devices. If they open up to other payment methods, my subscriptions are no longer centralized, I have to give my credit card information to more parties of variable trustworthiness, I have to worry about subscription renewal policies for every individual app, I have to figure out different methods of cancelling which could be a more difficult process than hitting "cancel" and trusting Apple will stop the payments, etc.