For published works (say, a blog), discoverability is probably a good thing. For communities, however, with many-to-many communication (forums, etc.), discoverability is an antifeature. Community building requires some degree of common ground, which obscurity naturally filters for.
The other downside of mass-popularity is that above a certain scale, your community becomes a target. Both for individual bad actors (spammers, vandals, etc.) and for the apex predators of the small community world, commercial interests. Look at Maker Fair transitioning from a relatively niche convention of people showing off their cool stuff they made, and some miscellaneous sponsors and vendors looking to appeal to those people, to an over-commercialized affair with a thousand people trying to sell you a 3D printer, because that's the big moneymaker.
Community norms are what makes spaces worth inhabiting, and they just don't scale well.
The other downside of mass-popularity is that above a certain scale, your community becomes a target. Both for individual bad actors (spammers, vandals, etc.) and for the apex predators of the small community world, commercial interests. Look at Maker Fair transitioning from a relatively niche convention of people showing off their cool stuff they made, and some miscellaneous sponsors and vendors looking to appeal to those people, to an over-commercialized affair with a thousand people trying to sell you a 3D printer, because that's the big moneymaker.
Community norms are what makes spaces worth inhabiting, and they just don't scale well.