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.
Yes and even if the El Salvador have accumulated most of the remaining bitcoin, that won't still make them the richest country in the world. Being dependent on the bitcoin alone won't be enough to at least be a part of the Top 5 richest country in God's green earth, they should have a lot more to offer that will make their country grown in the years to come, which they don't have. Also, 8 years is to short, I reckon it would be at least 2 decades from now to see their improvement.