They can move to Discord, Whatsapp (huh!), Signal, Reddit or some sailor forum.

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.

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.