Apple can easily say "Use our store exclusively and you get our security/privacy guarantees. Go outside our store and you're in the wild west". App developers can then decide how much fee they are willing to pay for access to the user base who refuse to venture into the wild west. Other stores might try to persuade users that they are more secure and more private too via stricter review policies or more locked down permissions etc.
It would prevent fragmentation and give people a choice to pay up if they actually value Apple's extra protections (if any).
If the EU has the right approach, then they still do not have the right implementation.
The only reason they haven't been smacked with a billion dollar fine for their Bullshit (Core Technology Fee)yet, is because of Trump.
*And anything on their payment rails should have a normal transaction fee, e.g. Stripe’s retail rate is 2.9% + $0.30.
The per-user model means that apps which have adopted freemium and advertising-driven models wind up having quite different financials, and could be more expensive.