On a global level, El Salvador will be closely monitored by all other nations. The bigger economies will see this as an experiment in order to either adopt or shun bitcoin completely. It is both exciting and scary at the same time.
I can agree with you that El Salvador will be under a special magnifying glass when the BTC becomes legal tender (which will happen in about 90 days), and that the country will serve as an experiment and role model for all other countries that may have considered moving in the same direction. This country is not the only one that has a problem with the national currency (which it doesn't really have because it uses the US dollar), and if it turns out that Bitcoin can effectively serve as a currency, we can expect other countries to introduce BTC as a legal tender.
Countries like the US or GB and certainly China or India will never give up their currencies even in the way that they will add another currency as an alternative - because that would mean they think their currency is not good enough - but I don't think they will prevent other countries to experiment with Bitcoin or any other cryptocurrency. As you say, perhaps this is an opportunity for small nations to become less dependent on others - and if there is little truth in rich countries wanting the progress of less advanced and poor countries, then let them try to do something for themselves.
if I'm not wrong Japan also legalized bitcoin, but it's still a drop in the ocean.
What Japan and Germany did a few years ago is not the same as what El Salvador did - it's something like
"Bitcoin is legal in our country" compared to
"Bitcoin as a legal tender - which would mean - everyone has to accept it as official currency". More about that can be read
here.