I don't necessarily agree that accessibility is a universal good thing, at least when it comes to building a community. Reddit is a good example - there is basically zero effort required to join and post. There is also very little incentive to remain active and post good content (karma is more or less irrelevant compared to incentives in forum sites of the past.)

As a result, you get fairly low-quality discussion content and essentially zero sense of identity - both of which were not a problem in pre-centralized niche forums.