I'm afraid that Bitcoin becoming a legal tender currency worldwide, would only be a pipe dream. As long as it's not mandatory for mainstream use, merchant adoption will remain low. Probably forever. It's this reason why Fiat will always be the preferred choice for payments. At least, we're free to be our own banks with BTC. You can either use it as "Digital Gold" or as a currency. The choice is yours. One thing is certain, and that's demand for Bitcoin will continue to rise "for infinity and beyond".
If we dare to face the truth, the idea of bitcoin becoming a legitimate global currency is really a pipe dream. Governments will never accept a currency they cannot control and they have no shortage of better solutions. There is no reason for them to favor a currency they did not create and cannot control.
Additionally, bitcoin has too many limitations to be a currency as it can only process 7-10 transactions per second, which is too slow compared to traditional payment systems like Visa or MasterCard, let alone compared to altcoins.
Given Bitcoin's decentralized nature and security, it is more suitable as a store of value, an asset like gold.