I want to agree with you, but Planet Minecraft is still incredibly active so I guess it boils down to having good marketing, active engagement with the userbase and regular improvements.
That whole deal with MCF being supposedly shut down probably steered some veteran users off the forums who never returned, despite that never coming to fruition.
it's been waves of that but honestly minecraft isn't that popular with people who typically go on forums any more. those people are already on other forms of social media. it's just gonna go the way of irc eventually, which is a bummer. dang zoomers. not sure why pmc is still going.