I guess governments may indeed shift to the use of CBDCs, so that's roughly the equivalent of stable coins. They'd be unlike the existing stable coins because they're not backed by fiat currencies; they're the fiat currencies themselves.

However, if this happens it is impossible that governments will issue their digital coins on a platform they do not own themselves. I don't think governments will make use as their official currencies coins which are built on Ethereum and other private or, if there are any, decentralized and independent blockchains.   

So I still don't know how you would possibly earn from hodling stable coins.