This is an important thing to consider, and why NodeBB decided to even pursue federation at all.
It's arguable that we've reached the point at which forums cannot organically grow due to the ubiquity of social media. Depending on who you ask, we've reached that point 10+ years ago already.
It's becoming increasing imperative that forums federate or risk dying due to attrition. Forums used to be the social network for niche topics. Facebook (with Groups) and Twitter (with hashtags) started competing, and Reddit (with subreddits) made another huge dent.
There are some communities that fear integrating with AP will cause their local communities to become flooded with just anybody. Those fears are unjustified, but understandable.