In theory, I agree with Alex: in an ideal world, we would have tens of developers supporting Wikisource (paid by the WMF, by the chaptes, by GLAMs), we would have many rich communities, and we could surely imagine a new structure of all our websites would that allow us to store books in the same place, and at the same time have different village pumps and interfaces and gadgets etc.
Unfortunately, for the time being, we hare scattered communities with no software support whatsoever: we don't have a Proofread page for Right-to-left languages, just imagine how much time it would take to redesign a MediaWiki for being a multilanguage, unique digital library, in which anyone can contribute.
I try to be more concrete and think about smaller goals we can achieve right now: maybe, in few years we can rediscuss this :-)