Yes, legal currency is now very convenient to use, and it is still the mainstream of social use. But when Bitcoin was born, a whole new choice was given to the people. People can use Bitcoin freely without control. This kind of consensus idea is constantly spreading. As the underlying infrastructure of the Bitcoin ecosystem gets better and better, the consensus of Bitcoin will become stronger and stronger, and the use of Bitcoin will become higher and higher.
Wouldn't it still depend on how much "convenience" Bitcoin could actually provide and whether said convenience is, well, actually convenience to the party using it? Like take, for example, Bitcoin's ability to transact worldwide without borders and problems and whatnot compared to actually using fiat from one country to another, who says that said problem wouldn't be addressed in the future right? The only real way imo for Bitcoin to actually be the main currency to be used is if Fiat was complete. I hardly think that people would suddenly turn to Bitcoin just because others said its good, when in their current situation, Bitcoin and fiat pretty much does the same thing that they always need it to do.