Ah, the forced $50,000 per Bitcoin in 2017. Let's see what this means.
If miners decide to sell all their mined Bitcoins, (or 50%), this is how much daily fresh money is needed on the exchanges for the price to be stable:
2014-2016 : $180 million ($90 million daily if 50% sold)
2017-2020: $90 million ($45 million daily) <--- look here
2021-2024: $45 million ($22 million daily)
etc.
Don't forget that they have to sell to cover their electricity costs. So, if Bitcoin is $50,000 in 2017, good luck in finding that daily fifty million dollars.
It's an interesting point.
But then, at 50,000 a coin, with around 15,000,000 coins in distribution (for the sake of argument), that gives a market cap roughly equivalent to the annual GDP of Benin or Moldova.
IF (and that's a big IF) Bitcoin does start really taking off worldwide, then 50m p/d is nothing. Forex sees around $5 trillion traded a day - around 0.2% of the daily Bitcoin USD trading volume in mid-Jan.
I know, I know. These numbers aren't direct comparisons, but it helps to put these big numbers in perspective now and again.
More meaningless numbers for comparison: US from China imports around 1000 millions $ worth of stuff per day. 50 million is 5% of that.