I wouldn't discount their success and future success (as much as I'd like to).
What's odd is they haven't stopped growing. It's just been linear user growth.
They could literally do nothing and the site would still grow. It's closer to Wikipedia than a social network in some respects. And by that standard, we should try to judge it.
I look forward to the next forum site as much as the next person. But I don't know what it would look like. If anyone does, I definitely urge you to build it if you can.
Substack has every incentive to build a community. Discord as well. So there's definitely opportunities, but from where I sit, I don't have any insight as to what you could do to beat this behemoth.