i don't see it that way. bitcoin doesn't have to replace fiat to be considered "gone mainstream". the bitcoin adoption, or the ultimate one mass adoption is when people have the option to pay with bitcoin if they choose to or continue using fiat if not.
and i have never read anywhere that Satoshi said anything about "replacing". even in the paper it is referred to as something like an alternative method.

as for advantages, i'd say the main thing is the control and power one has when they use bitcoin compared to using fiat where they have to rely on banks, credit card companies,... to have access to their own money. and that comes with a lot of limitations.