Bitcoin will not replace dollar soon. At least not for the next 20 years. The user base of bitcoin is too small and governments will try their best to curb the usage of bitcoin.
It cannot, and should actually not replace any currency because we all actually need fiat unless Bitcoin gets used completely as a currency everywhere...
Whether or not, still most people in the world know what Bitcoin is, so even if governments try to ban Bitcoins, people will still use it...
I do not think that the bitcoin is going to replace the dollar because the dollar also has paper money and bitcoin does not have enough at least.
It would be so weird if the bitcoin would take over the US Dollar.