> Also, swapping in a new registry for your base images is really easy.
This is the exact problem! Sure, MySQL, PHP, JDK, Alpine and other images would probably be made available, but what about the other images that you might rely on, but the developers of which might simply no longer care about them or might not have the free time to reupload them to a new place.
Sure, you should be able to build your own from the source and maintain them, but in practice there are plenty of cases when non-public-facing tools don't need updates and are good for the one thing that you use them for. Not everyone has the time or resources to familiarize themselves with the inner workings of everything that's in their stack, especially when they have social circumstances to deal with, like business goals to be met.
In part, that's why I suggest that everyone get a copy of JFrog Artifactory or a similar solution and use it as a caching proxy in front of Docker Hub or any other registry. That's also what you should be doing in the first place, to also avoid the Docker Hub rate limits and speed up your builds, not downloading everything from the internet every time.
Otherwise it's like saying that if your Google cloud storage account gets banned, you can just use Microsoft's offering, while it's the actual data that was lost that's the problem - everything from your Master's thesis, to pictures of you and your parents. Perhaps that's a pretty good analogy, because the reality is that most people don't or simply can't follow the 3-2-1 rule of backups either.
The recent Facebook outage cost millions in losses. Imagine something like that for CI/CD pipelines - a huge number of industry companies would not be able to deliver value, work everywhere grinding to a half, shareholders wouldn't be pleased.
Of course, whether we as a society should care about that is another matter entirely.