A lot of things exist in emoji because of early adopter advantage. A lot of the money stuff is either dollar or yen for this reason because they were first to the table. It's the same reason there are also so many other non-money emoji that represent quite uniquely Japanese cultural concepts in there.

There's a heck of a lot of world currency missing from emoji, but I would never suggest adding them all in. Everyone across the world is quite happy to use emoji with $ on as a synonym for 'money' regardless of what currency they actually use, so that already gets the job done as "generic money symbol"

It works because $ is so culturally ubiquitous.

When it comes to designing a new emoji that represents 'digital currency' we could take a similar approach as with fiat currency and just pick the most well known one (BTC) and roll with it.

But my preference would be to do otherwise, and design a non-specific emoji which represents the concept but without just being an existing logo.