One reason might be because this gives the US too much influence over the world (I know I am oversimplifying here). As long as everybody got along well, and most of the countries in the world were growing economically, the dollar served a common goal. But now we are increasingly heading towards a world where trade wars are common and policy makers are looking inwards rather than outwards. Perhaps all the armed conflicts that US has gotten into also lead to more ideological misalignment in the world.

So, in such a world, I wouldn’t find it so surprising that certain countries would like to wean themselves of the dollar. Of course, this it is more important for powerful geo-political actors like China and Russia, than Somalia (uses USD extensively), and as an example you can look at how China is planning on offering a yuan priced oil contract that is backed by gold.

This is not logical. Just because something hasn't happened doesn't mean it can't happen.

But it’s a pretty far-fetched hypothetical scenario. You could also ask, what would happen if all of a sudden everyone in the world decided that food had no value? Gee whiz, all the farmers and restaurant owners would be screwed!

I think it's about as likely as somebody dropping something and it falling up. Keep testing the existence of gravity, it could change on you.

I'm not sure that you realize that the current monetary system is only about half a century old. Fiat currency, backed by nothing except the government, has only existed since the 1970s.

That's the same reason the world won't suddenly decide that BTCs are worthless.

But most of the world doesn't trade in BTC, so most of the world could easily decide Bitcoin is worthless without a hit to their own pocketbooks.