This writeup is useful for backend engineers: https://atproto.com/articles/atproto-for-distsys-engineers
The simple answer is that atproto works like the web & search engines, where the apps aggregate from the distributed accounts. So the proper analogy here would be like yahoo going down in 1999.
Sorry, but this analogy is very misleading, no one browses websites through Google's servers.
For example, right now in my URL bar I read "news.ycombinator.com", not "google.com/profile/news.ycombinator.com".
If Google goes down now I can keep browsing this website and all the other websites I have in all my other tabs as if nothing had happened.
Google and MSN Search were already available at this time. Also websites used to publish webrings and there was IRC and forums to ask people about things.
This is a fantastic write-up, thanks for sharing!