AFAIK you can still access third party app repositories such as AppValley or TweakBox on iOS without jailbreaking (you have to install/trust their configuration profiles however).

Apple could be forced to allow another app store, which is essentially what the EU is demanding here.

> Apple produces all of its own devices and it's impossible to install another app store on the phone without jailbreaking

Not completely true: corporate users can get special distribution licenses to be able to distribute to their employees. https://developer.apple.com/programs/enterprise/

A corporate internal distribution license is really stretching the definition of "another app store." I think most people on the sidewalk would only call something an app store if it's open to the public.

The only thing stopping you from installing this on everyone's phone however is the Apple secret police and the license agreement you sign with them. Technically it's possible with the right certificate signed by Apple.