Let’s assume that stablecoin usage grows massively. Everyone everywhere has easy access. 

People in countries where the currency is less trusted will move lots of money to stables. 

What happens when these less trusted countries lose control over their local currency?

IMO, if crypto is ubiquitous, it will make adoption of small country currencies *easier*. Because I can pay you in whatever I have, and you can take your payment in whatever currency you want. Small governments can ask merchants to take payments in their (digital) fiat, pay taxes on it. tourists will use it and not even know.