Largely the same as if they just disappeared and stopped logging in. The community would become abandoned and unmoderated.
Reddit had a system where you could request banned or inactive communities, Lemmy instances don’t usually have this, instead requiring you message that instance’s admins directly.
That’s why you shouldn’t buy books from Amazon or other online ebook stores instead just download the ePubs elsewhere.
I’d also highly recommend KOreader if you have a Kindle or Ereader which supports it, as it supports many more formats and has a nice interface.