Countries that use the USD for their official currency, and are not a part of the US, have a broken financial system. They need to fix that to a point where they can implement their own sovereign currency.
Maybe it's not as simple as you think? These countries generally chose to use USD, do you really believe that they all just made a stupid decision with no upsides?
You are right. It isn't easy. But no one ever said running a country would be easy. If you are choosing to use another country's currency for your economy, you carry all the risks associated to that currency
In the case of El Salvador, they adopted the US dollar when their native currency collapsed after a bloody 12 year long civil war. Notably the civil war was covertly funded by the United States in order to support regimes aligned with US interests. At the height of the war, the US was funneling $1-2 million USD per day (1980 dollars) to fund an army of child soldiers. [1]
The US is a proximate root cause for the loss of ES’s currency. I could see why they would want an alternative to the USD as legal tender.
In general the USD or the euro emerges as defacto form of legal tender on the other side of some sort for societal collapse. It can be a temporary thing, but it is not a healthy thing.
Countries that use the Euro for their official currency, and are not the European Union, have a broken financial system. They need to fix that to a point where they can implement their own sovereign currency.