I still think that an important aspect of Instagram/Facebook/Reddit etc. being the default spaces is that that’s where admins (including non-technical admins and moderators) choose to set up and develop their communities in the first place, in the case of Facebook, Reddit and Discord for example.
Registering and using Discourse as a user is already much easier than setting up Discourse as an admin. I have a lot more trouble convincing other non-technical moderators/community managers to choose to set up a Discourse forum over other platforms, than to get people to sign onto existing Discourse forums.