Are you only allowed to affiliate your Apple ID with one country’s app store? If I moved to Japan and switched to the Japanese app store would I just lose all of the apps I purchased via the US store?

Yes, you can only have your account associated with 1 store.

You won't lose the apps you have installed but

1. They won't update until you switch back so you'll likely need to switch 2 to 4 times a year as your bank and/or other services deprecates their old apps.

2. Switching requires manually re-entering your name, address, phone number, and credit card info every time.

3. Any subscriptions you have via Apple Pay will be cancelled.

Thanks Apple!

And note: While the average person does not have bank accounts in 2 countries I've run into plenty of apps I needed as a tourist that are only available on that country's store. For example the Lime app is not available in the Japanese store because Lime isn't in Japan but if I'm visiting a country with Lime then I want to be able to use the Lime app so I need to switch stores.

Meanwhile my Switch lets me have multiple user accounts. Each user's account can be assigned to a different country so I can buy The Touryst from the USA store and Japan exclusives from the Japan store. (not saying 2 accounts are the best solution it's just surprising that a Nintendo device does this part better than Apple)

That's more of a developer problem than an Apple one. Most of the apps on my phone work in any country around the world. I've used Uber in many countries, Spotify, Netflix... If your bank wants to restrictively localize their app distribution, how is that Apple's fault?

It is reasonable for a company not do want to deal with people outside their country including payment systems they don't support and laws they don't understand, taxes they have no experience with etc...

It is not reasonable for Apple to make using the phone in more than one country so hard. That is something Apple has the easy power to fix.