IMHO HTML documents backed by a versioning system (probably fossil or pijul rather than the overly complex git) are the way forward for documents where content is much more important than presentation.
While “text in a VCS” is a great option, it’s obviously far less usable than something like Google Docs, and you still don’t get real-time collaboration, which can be really nice.
Yeah... I'm wondering though, Fossil is based on SQLite - a database - and databases are designed to solve the issues arising when multiple users try to change the same data. (Also, fossil by default works in "autosync" mode.) So it should be "easy(er)" to make a real-time collaboration tool based on Fossil ?
P.S.: By researching this, I've stumbled on a (barebones) alternative to Google Docs : HackMD/CodiMD/HedgeDoc :
https://demo.hedgedoc.org/
This isn't a small thing for many users.