I think one thing you're missing here is that under such a system the defaults would likely become your locally hosted /c/books rather than the largest one. Even still you'd probably see posts from the largest books communities because /c/books@your_instance follows multiple /c/books@big_instance. Community blocking would likely still work as it currently does so any books communities that you were not fond of could still be blocked.
There is still the issue of where do you post and I think the answer looks something like:
- Post in /c/books@your_instance if you want to talk to your neighbors
- Post in /c/books@big_instances if you want to talk to everybody
Which is more or less how most people would decide where to post book stuff anyway.
I think you can be certain that preparations have been made. Whether it'll be enough is a different question