Alternate app stores is a matter of developer choice, but also consumer choice. If you want to pay an extra 30% for everything then that's your choice...let the rest of us choose to not do that if we want.
It's more of a question of who do you want to pay 30%; or for smaller developers 15%.
I suspect many smaller developers will find alternate app stores no cheaper and not very attractive due to a small user base. Big companies like EPIC could run one and probably will, if just to spite Apple, but I doubt they'll give developers a much better deal overall.
As for the App Store, Apple will find ways to recoup any revenue drop.
If you think that Europe is just one country like the US, you are mistaken!
Actually, the US is not, in many ways, one country, it has states, territories, sovereign nations all within in it; all with their own laws and courts, for example. Some of those territories citizens aren't US citizens, some are. The primary difference is one of a federation vs confederation, with various strengths and weaknesses.
Of course, as Brexit proved, you can leave the EU while SCOTUS determined succession was illegal after the Civil War.
It doesn't matter, from where the web site is originated, but if it can be opened in the EU, it comes under the rules and regulations of the EU.
The EU law may say that, but that does not mean a company with no presence there needs to follow it.
The transnational nature of the internet is one of its biggest challenges, with conflicting regulations. The EU says that any website opened from the EU establish a presence in the EU and thus you must follow our laws; another country could similarly argue an EU website available in our country is subject to their laws, and fine them for violations. Collecting is another issue. Just don't have an executive visit.
Some of this can be addressed with treaties and in some cases companies simply exiting a country rather than comply.
Maybe, most web site owners don't want to waste money releasing one for the EU, and one for the rest, they might just place the cookie acceptance popup as default. So, people in the other countries also get that popup. Does that happen with you, if you are outside the EU?
No, it typically only happens for sites originating in the EU. When I am in the EU, some major companies simply block EU access rather than worrying about compliance. Other sites simply ignore the law since I don't get popups on many; but then again they have no physical presence in the EU.