I agree about the idea that bitcoin is an alternative currency to fiat and other legal tender, but over time bitcoin may become the main currency in some countries that do not have their own currency. The real use case of bitcoin as a currency is basically not going to threaten the existing financial system, bitcoin is just an option if users need it.
I'm not sure that Bitcoin will be the main currency some countries. It seems to be an alternative currency only. A country needs a currency that can be printed and it is owned by the country itself. And a country shouldn't expect to have a main currency with too high volatility. Also, I assume Satoshi never dreams to make Bitcoin to be the main currency, he only means to create an alternative currency.
CMIIW
I think that even if bitcoin is adopted as a legal tender, fiat transactions will still dominate. Not everyone wants to use bitcoin as a means of payment for various reasons, so only a few people will use it. I think the same thing happened in El Salvador, some people still use fiat even though bitcoin has been legalized as a legal tender.
Indeed. Even if Bitcoin is a legal tender, fiats will remain the most options for any transaction in a country. Sure, people keep using fiats in El Salvador even if Bitcoin is declared as a legal tender there. Using Bitcoin requires stable internet connection, it some areas this is still something impossible. Especially in remote rural areas.