Stack Exchange v1’s business model was “build your own community, we’ll give you the software on a subscription basis”.
That model failed because communities require critical mass, and achieving that is really hard. All but one community created at that time (Math Overflow) got any significant traction, if I recall correctly. SE eventually decided to shut down the SaaS model and to launch instead their own communities via Area 51.
Discourse compares itself to the wordress.org model because it’s open source and anyone can host their own, but aside from the self-hosting part, the model resembles SEv1 a lot more than WP. There are a lot more solo bloggers out there than forum organizers, and they can start blogging (and paying) from day 1, without needing to first build up a community with critical mass to make it something they can use.
So what’s different this time around?
Btw, I’m assuming that charging end-users for hosting their forums is a major part of the revenue stream, because if the plan is just selling to big enterprises I don’t even see why charge end-users in the first place.