Sure, but the cost of pre-planning for an event like what Reddit is attempting is high - not just in terms of money, but in terms of how large you grow your com

Sure, but the cost of pre-planning for an event like what Reddit is attempting is high - not just in terms of money, but in terms of how large you grow your community, since (for example) getting someone to join your subreddit is infinitely easier than asking them to sign up for your forum. Reddit lasted what, over 10 years? And Discord started in 2015 but still seems to know not to destroy communities by e.g. making third-party moderation bots pay to use the service. It makes a lot more sense as a community to coast on whatever the current most popular free platform(s) are, since it means there's no financial overhead to running the community and the centralized nature means the barrier to entry for prospective communities members is near-zero. Once federation can achieve these things, we'll see communities at least fracture with an arm on the federated site, ready to jump ship once Discord or the next platform starts to sink.