Why does Amazon even need to have a sunset date (or perhaps how is it they can even define one?).

They do because why not. Its their hardware and software. You chose to be bound when you purchased and agreed to their Terms & conditions.

Or do these 2010-2025 era Kindle device still get updates for tings like DRM?

It's literally written in the article. You can read existing books as long as you do not factory reset the device or de-register your account. New books cannot be added or purchased.

The Amazon store stops working, you can still move either EPUB or the AZW files via USB, can you not?

Unless, amazon provides a DRM free Epub at that time. The answer is no. You bought the license to read not own the ebook. Maybe, look into tools to download and decrypt the books if you have some.

I would recommend getting your Ebooks DRM free from other places. Do not be restricted by corporations who exploit for profit.

Yep! You need a program to get them off of kindle, to put anything on you just need to find the right folder. I have been using Calibre for years, it's great. Does all the work for you and is easy to use.