If fiat was an option and not a stipulated legal tender, then a lot of people currently depending on it would not and would find alternate means to transact. For Bitcoin to go mainstream, 100% of the people do not have to use it, it just needs to be easily accessible anywhere and available as a means of payment by merchants globally. There would still be preferences by people who prefer fiat, altcoins or some other means of transacting.
I know some of you would say improvements of Security, Scalability, and User Experience, but aside from that, What is the most needed for BTCitcoin right now?
Security and to an extent, user experience is not a problem with Bitcoin. As one of the most secure networks it only relies on the holder to use it the proper way and the user experience depends on the wallet you use to store you coins.
Scalability is a work in progress and is one major barrier to adoption, along with volatility.