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.
Exactly. Stablecoins are private or corporate wealth, whereas CBDCs (if they happen) will be national wealth. The two will compete with each other for market share (but it's a different market share to that of Bitcoin and other non-backed crypto, because those serve an altogether different purpose), so holding corporate stablecoins could potentially cause you to lose money if CBDCs achieve dominance in that market. And if any given nation does launch a CBDC, they have the notable advantage of being able to legislate against stablecoins, so it probably wouldn't be a fair fight.