I agree with you, El Salvador is just a small country. They don't even have their own fiat, so the change wasn't too difficult. Everything was decided quickly by the president. Although they've been successful with rapid economic growth thanks to BTC, it doesn't mean other countries can do the same, especially large countries with strong fiat.
Yes, as compare to bigger countries El Salvador is actually small and could easily decide to make Bitcoin a legal tender but for other countries it might not be that easy to make Bitcoin a legal tender.
When it comes to strong fiat and large countries then US is the one that fulfills that scenario and I'm pretty sure adaption of Bitcoin or making it a legal tender could be somewhat hard for an economy like US or any other top economy.
In fact such steps could be helpful for Bitcoin itself not for those economies. However, as a Bitcoin holder I always want top economies to accept it as a legal tender.
Bitcoin is one of the best investment in the world and the best crypto coin in the world, Bitcoin has helped to grow the world economy and has also helped in making some individuals millionaires, and Bitcoin is a peace of mind investment.
A lot of countries will not accept Bitcoin because they feel Bitcoin will destroy the economy, some countries has already made the use of cryptocurrency illegal and will be arrested and persecuted if Caught making transactions in cryptocurrency.
Some countries has accepted Bitcoin but has not made it a legal tender but it will happen soon.
If Bitcoin is made a legal tender in some big nations of the world Bitcoin will become more valuable and increase in price, that will be a big one for Bitcoin and it's investors.
I believe Bitcoin will soon become legal tender in countries and will increase in price, a lot of people believe Bitcoin will not grow more than it is now and that is why they are not investing in it but like those that had opportunity to invest in Bitcoin 10 years ago and refused are regretting now that is how people with this thought that Bitcoin won't grow more than this will regret.
Bitcoin has not reached half of it's growth so it will keep growing and Bitcoin will make history.
When nation's will see the way the world is going and the way Bitcoin is becoming recognized and used by many they won't have any choice but to start making it a legal tender.