Sorry I misunderstood what you were saying.
As far as maintaining a private fork of Discourse, here’s the difference:
Discourse has several people employed full-time to add features and fix bugs (including security bugs), as well as a large number of fantastic contributors from all over the Internet. There’s a pretty good chance the changes in a private fork is going to end up conflicting with the changes made in mainline fairly often, necessitating manual review and modification of the local patches.