Imo this was just a media stunt to gain attention. You can't seriously expect Bitcoin to reach 500k in three years 
Actually, if the growth went "down" to 4,5x per year, the price would go from 6000 to 27000 to 121500 to 546750 in 2020. And exponential growth actually has this peculiar attribute that increase the rates of gain, so if an exponential trend continues the price would be well over a million. It's more of a question of how much longer the adoption will remain exponential.I definitely wouldn't expect it to be any higher than 20k to 60k by 2020 though and wouldn't even plan my life around that.