The world moved on from the gold standard briefly at the start of WWI, but then returned. Then, during the Great Depression, the need to better control their currency and do large financial transactions for which they simply didn't have enough gold spurred most countries to move away definitively from it. This spurred recovery from the Great Depression and allowed huge investments in needed work.

All of a sudden, it became clear that well functioning economies could reliably promise money that would become available tomorrow, and use it to start getting there today. This is a major boon to any country, and having to save up resources until you can start huge infrastructure projects would be a return to terrible times.

Later edit: As for the claim that El Salvador would use the Lightning Network, I highly doubt that the citizens of El Salvador will each have a bitcoin wallet and lightning channel connected to it. Much more likely, some citizens will have some account with a bank (exchange) that will facilitate payments through the LN or whatever other mechanism they chose to use internally, that no one will really care about.

Of course, with bitcoin as volatile as it is, no one in their right mind would pick up a credit in BTC, so the greatest need for fractional reserve banking will be far away. Even so, if people start buying heavily into bitcoin, I wouldn't be surprised to see banks/exchanges start to offer BTC that they don't yet own.