Looking at the chart on how the price went up, it was just the end of this year it suddenly peaked. The price from previous years looks like a very slow rise.

Now on the speculation on how it will increase more within three years, it may not be surprising to reach that much.

End of December 2016, price of bitcoin was only around $960, End of 2017, it may be still around $10K. So in a span of 1 year, the price multiplied by 10. If the growth continues that trend, every year multiply by 10, it may reach $1M by 2020.

Maybe that was the model he used that is why he speculated that much.

I think it will reach around 100-200K by 2020, it might plateau its price or will go on slow rise as how it was in the previous years.