Hm, one of the things that effectively could prevent BitCoin to replace or even 100% complement existing FIAT :
People don't understand money, they can only use it. For as long as that works (even with inflationary side-effects and all the implications of using "Legal Tender" debt notes), they can't even be bothered to understand it.
Sounds harsh, but monkey don't care. Monkey function. Monkey working. Monkey consuming. Monkey dies, offspring takes same place. Rinse, repeat, cycle.
Given that state of mind, I can't imagine the average people would ever get their heads around BitCoin or comparable for it to achieve "money" status in their minds.
That would take a full-blown launch of state/mass-media advertising campaigns.
The internet will certainly (and already has) educated a certain percentage of people to allow them to use it, but that's about it. It has limits, which would need to be overcome (the most realistic way would be mass adoption by everyday "around the corner" shops worldwide).
It would certainly require a
massive education of folks concerning internet/IT security as well. BIG time that. Not sure "joe sixpack" would be able or willing to process so much knowledge which additionally is dynamic (needs to be kept up-to-date).
To me, in that respect it looks like BitCoin is like a functioning raw engine with all required basic tools/instruments/gauges attached. Usually works like a charm, but it's still a raw design like a prototype - some assembly requried, batteries & instruction manuals not included; available scattered across dozens of libraries.That's not what people look for, money needs to be as easy to use as opening a beer can or unscrewing a water bottle.
Could be BitCoin needs a nice, dumbo-easy, one-size-fits-all-wrapper that gives a User Interface even joe sixpack or a 5yo kid can use without much brainpower involved.
That wrapper (software & actual hardware) doesn't exist so far.
Call it an "Accessibility Assistant" that allows non-internet/non-techy persons to access & operate beneath-the-hood hightech gear - like a smartphone most people couldn't naked program if their life depended on it - but give it a nice simplifiying Operating System and User Interface - and today youngest kids can (and love to) use them. Such a bridge technology would be required to close that massive usability gap (function vs. usability/accessability).
Simplify, simplify, simplify! - Henry Thoreau, 1817-1862
PS.
I already wrote about some other unresolved (unresolvable?) issues that would speak against it in another thread, if interested :
https://bitcointalksearch.org/topic/m.4919602(3rd paragraph, others mostly off-topic to the OP)