For starters, that's not going to happen, that we don't exchange bitcoin for fiat. That was imagined at some point years ago but I see it as a long shot. First, because bitcoin is valuable and will become more and more valuable and scarce, so people will tend to want to save it instead of spending it. And second because there are going to be a lot of alternatives to CBDCs and shitcoins and payment systems for day-to-day payments. Add to this that bitcoin as a hypothetical global payment system would have the problem of scalability, which is not clear that it can be solved.

In short, the scenario you suggest is not going to happen, at least at the global level. If there are places such as El Salvador where it is declared legal to tender, then you will not have to pay taxes for it, but where it is not declared as such, you will have to pay taxes as it happens now and Lucius has explained well.