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 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.
In contrast to countries that do not have official currencies so that the use of Bitcoin as a legal currency can be applied, but it is not possible for some countries that have their own currencies because they have to change the rules in advance for the application process and this is not easy to apply.
The idea of an alternative currency is an instrument of the process of achieving Bitcoin because Bitcoin is made not to threaten the financial system, but as an option for convenience.
Bitcoin began to be seen as an instrument based on the accuracy of use and how the idea began to be applied by El Salvador who openly received Bitcoin as their official currency.
For the future process we never know how things that are impossible can happen, but more and more countries that apply bitcoin as an official currency will actually make limits with the amount of Bitcoin availability itself.