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!