My suspicion is before we get anywhere near 2 billion people using bitcoins that there will be factors at play that will dwarf the impact on price arising out of adoption and its use in day-to-day transactions.
My fear is as the realisation dawns on the bigger players that Bitcoin is 'the one' that it will be pumped by banks and central banks who can magic large sums of fiat into existence with which to buy bitcoin (and who may even dump gold temporarily), creating the ultimate bitcoin megabubble so that billions of latecomer ordinary people and businesses buying their first bitcoins will be shafted as the great bitcoin dump starts, into pms, into land, artwork etc. into a 'better' crypto - assets that had been neglected for the period bitcoin was so attractive, even back into fiat (they could use this point to launch more 'stable' 'new crypto-dollar or new crypto-pound etc.) until bitcoin finds a stable long-term pricepoint well below (even orders of magnitude below) the hyped price the masses bought at. Problem is of course nobody beforehand can know what this price will be. We would still be in a much better world than today because with bitcoin established fiats (old or new) will not be that attractive but if central banks are deliberately hyperinflating forcing people into crypto on their pump stage people could end up with 1000th of what they had before.
Btw, I'm only just exploring this idea so please point out if there's an obvious reason this is not how the future looks!
Unfortunately this scenario is almost a certainty if Bitcoin achieves mass adoption, such is the nature of financial bubbles. I don't see any effective way of preventing it. Caveat emptor.
By now pretty much everyone in the world has at least heard of bitcoin and has probably heard some 'conspiricy theorists' about how corrupt banks are and how corrupt the financial system is. If they still want to hold on to their fiat, it's their own problem, let them keep their trash while it lasts.
It is not that simple. I cannot invest the majority of my assets in bitcoin even if I wanted to (I don't). Reason- 50% in various retirement accounts and 20% in real estate. I am sure the majority of folks cannot invest in BTC in their retirement account(s). When winkelvoss or secondmarket ETFs will start trading, would you consider them fiat or bitcoin? I would say that it will be rather fiatty, but could be beneficial in nature.
Regarding investing bitcoin in anything right now-I think that it is premature for most (i would probably thought otherwise if I invested into thousands of bitcoins at single digits). Why? Because the rate of return on bitcoin appreciation is so far has been vastly superior in comparison with any business return, which suggests that you should invest fiat right now in businesses that produce bitcoin, not vice versa.
Also, even though many people will at first become poorer for it (all the wealth for the first-movers has to come from somewhere) eventually the wealth will become more evenly distributed because unlike the current system there's no one who can just magically pop currency into existence. So once you spent a bitcoin, you spent it, and someone else has it. You'll have to earn more bitcoin before you can spend it again.
Perhaps BTC was in essence designed to suck in and eventually distribute all that fiat excess that was printed in 2008-2014 and teach people being prudent with their money later on.