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.
Asking people to re-learn the new modalities and UIs and where everything is etc.. particularly for a less technical crowd.
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)
Further the success of Facebook was arguably the biggest contributor to startup culture ever - I would expect we'd have seen a fraction of the growth in VC if Facebook had never come to pass.
Groups, WhatsApp, etc, would be replaced overnight with, at least initially, a worse version. More hacking, probably worse moderation at scale, worse accessibility, etc.
Meta also gentrified East Palo Alto, and the Zuckerbergs now own a substantial amount of real estate in Redwood City and elsewhere. They've made a big footprint on the peninsula that deserves credit for the now $8 lattes in my hometown.
That being said, I've already cut Facebook out of my life years ago for sanity. So really, I'm just mourning what's already all but gone in my life.
That's about all I got.