I doubt that El Salvador will be the richest country despite making BTC as their legal tender.

Remember that there are several factors that you must consider on how you define a country's richness. Just because they can potentially acquire tons of BTC does not entirely mean that they will be the richest country. Unfortunately, a country can be rich if they are placed geographically right on the Earth. With their tons of natural resources, they may capitalize this advantage and turn it into richness and money.

While BTC may be the future of currency, this fact alone cannot withstand the other factor that may make a country rich.