Wonder how that will work for people like me that have an US iCloud account but a German AppleID for the Store
You might get either the best of both worlds or the worst of both worlds. Let us know after the change.
My opinion:
competition is always good. When there is no competition, there is exploitation of consumers.
On our Macs, we have the ability to buy from the Apple Mac App Store, direct from the developers, from various third parties, etc. In general our Macs possess the same keychains, etc as our iDevices. The world does not end because we have freedom to buy apps from about anywhere for our Macs. Our bank accounts have not been emptied. Our houses have not be sold right out from under us. This will be the same... but only for EU consumers.
EU people who like the "as is" or worry about the doom scenarios can keep buying exclusively from the Apple App Store, exactly as they do now. This doesn't force them to change from where they get iDevice apps.
Those wanting to buy directly from the developer or from a third party app bundler will have such options too. Those who want to buy apps that Apple has decided are not going to be permitted in the App Store will be able to buy them too. As an American, I will envy the great freedom the EU consumers will have in buying any software they want at market competitive pricing from wherever they want to get it.
Will some people in the EU install malware because of this greater freedom of choice? Yes... just as some people with Macs install malware. Is
everyone in the EU doomed because of this? No... no more than everyone with Macs all over the planet are not doomed by buying Mac apps from wherever we get them other than the Apple App Store.
Apple doesn't like this mostly because it endangers their very steep "30% right off the top" cut of nearly 100% of app sales. That is very lucrative cash made mostly on the sweat & efforts of third party app developers. Since Apple doesn't like it, fans are quick to spin all kinds of rationale in support of Apple. But much like iPhones having to go USB-C, all this "doom/disaster/all phones bricked/malware for all/etc" will be about as non-event as "lint magnet, broken tongues, wobbly USB-C ports" were with the new iPhone. Fans will make it sound like an end-of-the-world cataclysm for the EU... and then it will transition... and there will only be occasional incidents of actual issues caused by it... like the occasional post of iPhone hogging up all of that lint, wobbly or broken tongues.