There have also been cases where a customer violated Amazon's terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
I can't wait until a Senator or comparable "it's not a problem until it happens to me" lawmaker loses access to their digital library and goes on the warpath. That's the only way out of this "you will own nothing" hellhole we're in and moving deeper into.
Probably won't happen until Millennials and younger are in meaningful numbers in Congress or Parliament or whatever. A few Gen X politicians might be affected, but the rest probably don't have gigantic digital libraries of things they've "bought."
California is at least taking a step forward in legislating that "sellers" can't call it a purchase if you're only getting a revocable license. Shops wouldn't be allowed to use the word "buy" or "purchase" unless you get to own the product.
This is why they rarely pull your whole library, it's too noticeable and all these services have is public faith they're going to still be there. More often the case you'll just lose access to a purchase here and there and usually goes undetected especially if you have a large collection.