I think the most optimistic outcome for Bitcoin would be to return to its roots, which is to play to the nature of bitcoin as a currency, as the opposite of global fiat currencies, coexisting with global fiat currencies and ready to replace the declining credit of fiat currencies, such as overissued US dollars.
Bitcoin was supposed to be used as a currency, but there are some flaws with bitcoin that makes it almost impossible to use it as a regular day to day currency. We need to fix this issue without bring any risk to the security or changing the protocol. I doubt bitcoin will ever replace the current fiat currency anytime soon. The government around the world will not allow that to happen. The best we can see is that other centralized crypto currency (most likely made by the government) replacing the current fiat system.