where nothing is searchable, linkable, or otherwise legible. That's a tradeoff. It has some upsides, but downsides as well.
(granted search has been struggling to not suck in general lately, facebook among others has joined the campaign against legible links, so losses are taking place in web environments anyway)
Moving a community is much much harder than you're making it out to be. Especially when there's a long history of content and - most importantly - trust between members.
Asking people to re-learn the new modalities and UIs and where everything is etc.. particularly for a less technical crowd.
If FB disappeared they could reroute.
Bigger things are disappearing and we are going to be fine*. E.g. much world trade with the US.