For me, it is enough for Bitcoin to be legally accepted as a payment method. It doesn't have to be a legal tender to perform its function. If countries recognize Bitcoin as a currency, that would already be enough.
For Bitcoin to become a legal tender is a long shot, and could even just be considered as an extra mile. Bitcoin doesn't have to be a legal tender for businesses and customers to accept it as payment. I think it is enough catalyst for Bitcoin's adoption rate for a government to declare it as an acceptable currency.
I also think that Bitcoin don't have to be a legal tender of any country to achieve it's global currency aim, if it's accepted in any country as a payment option and it's business people will adopt it and accept it as a means of payment, then it'll still mentain it's function as a legally accepted payment method. I don't think that most governments will accept Bitcoin as a legal tender because they can't control it as a result of it being decentralized, the least they can do is regulate it. Bitcoin don't have to takeover fiat currencies, it can function perfectly well as an alternative digital currency, besides people are adopting it more as as an investment asset, because it's a store of value.