It could accept any credible currency if it was connected enough. Euros, yuan, rupees and yen aren't going anywhere for at least 20 years. Each one is a separate system and countries mostly connect to just one, which is USD, but that doesn't have to be the case forever.

India won't accept euros because it's not part of the ECB, not because it doesn't believe in their value. But India has accounts at US banks in dollars.

Banks do this, not countries. Most banks in the world have accounts at US banks to accept dollars with, they don't have accounts at eurozone banks to accept euros with, or Japanese banks to accept yen with. It doesn't matter in everyday practice because it's easy to exchange euros in eurozone banks or yen in yenzone banks with dollars in dollarzone banks. There's plenty of infrastructure for that. It matters in long–term economic trajectories because all those banks are holding US dollars and the US exports inflation to them and they're not holding euros and then ECB can't.