[tchocky]

Not really. It can still be a walled garden if there is an opt out option, so you can still be able to be inside it but with the option to go out of it and be able to sideload/use different app stores. Also the Apple app store will definitely still be the main source as people usually don't switch that easily for almost no benefit. No one will force you use a different app store as well.

[mantas]

What if super popular apps launch their own stores?

[ThatPlayer]

You can still choose not to use it. iMessage is super popular, am I forced to buy an iPhone?

Depending on your definition of 'super popular', both Android and iOS are super popular. Yet I don't need both of them do I?