It's inevitable (that Bitcoin will go mainstream), but fiat isn't going anywhere any time soon. Plenty of time for people to divest. I know I'd keep some USD in a USD side chain.
I think this is logically flawed. Because:
If bitcoin becomes mainstream:
1) i.e. a few hundred million users than the value of bitcoin must go up tremendously.
3) the value of bitcoin must come at the cost of fiat. In other words the value of the USD must come down significantly for bitcoin to go mainsteam
4) when fiat goes down while btc goes up, people will stop using fiat and use more btc. Because BTC is good for the individual people will more and more move to bitcoin.
5) at this point fiat money must collapse (its value) which increases this cycle. There is a tipping point (mathematical concept) at which point this may go over rather quickly.
I do not believe it is possible for fiat or bitcoin to exist side by side.
Right now we are not anywhere near a growth point that can enable this however as there is no real need for bitcoin today. the nr of transactions is pretty stable this year and usage is not going up much. bitcoin needs a killer app, something you can only do with bitcoin. at this point bitcoin is something weird and new for folks with a tech fetish or establishment haters.