Bitcoin doesn't need to be legal tender to buy things with it. That's just merchant adoption. Legal tender would be a govt designation that says the currency is the official currency of the nation. There can be more than one legal tender in a country, like El Salvador where both USD and BTC are legal tender.

But I wouldn't expect most nations to EVER make Bitcoin legal tender. Definitely not large countries that issue their own currency. It made sense for El Salvador because they don't issue their own currency, so they don't lose any economic control by adding Bitcoin as legal tender, and it was also a bit of a publicity stunt in order to try to make El Salvador a destination for bitcoiners around the world. But of course it also gives the citizens of El Salvador more confidence in using Bitcoin since the govt designates it as one of the nation's official currency, which is good for encouraging people to use it for remittances, savings, and of course spending, but its up to the people to adopt it over time.

I expect eventually there may be several smaller countries that adopt bitcoin as legal tender. But, generally, using bitcoin for payments will be done simply through merchant adoption. So basically don't expect to ever be paying taxes in Bitcoin in most places in the world, but do expect to eventually be able to buy most things with bitcoin.