The strategy has yet to make an impact as the countries economy is still not doing good. I feel it will take a few more years untill we see any positive impact. Another country that had hiddenly mined Bitcoin for sometime was Bhutan. They never declared Bitcoin as a legal tender but they plan to use those Bitcoin to boost thier economic activities by selling them when the price is right.
We can understand that strategy of El-Salvador will take time to produce desired results considering the current state of their economy and it might take several more years before we see tangible change in their economy. Regarding Bhutan, it was a smart move to secretly use their natural resources for mining Bitcoin, they have not declared Bitcoin as legal tender but they are selling them when the price is favorable. Bhutan aims to capitalize on potential benefits of Bitcoin without formally declaring it a legal tender. This reflects how countries are exploring opportunities being presented by cryptocurrencies.