So, here's some more fun with the numbers...one post asked whether it's simply the amount of gold that determines its value...the answer is no, it's the amount of gold (the supply) as well as the demand. Another post mentioned that the dollar would have a very different value in 10 years, but when we talk about a bitcoin being worth $200,000 in 10 years, we are talking about its purchasing power as measured in today's dollars. So, that statement means that in 10 years you would be able to purchase with 1 bitcoin what can be purchased today for $200,000.

So, let's suppose that people did eventually come to value bitcoin similarly to gold (there are plenty of reasons one could argue that bitcoin is a pretty good gold substitute, but I won't here). Based on the OP's numbers and assuming a worldwide population of 7 billion, the amount of gold held on average by each individual is ~0.76 ounces. At $1500 that's a value of $1140. If everyone we're to take half of that gold wealth and put it into bitcoin, that's $570 of gold and $570 of bitcoin on average for everyone on the planet. $570 * 7,000,000,000 / 21,000,000 = $190,000.

So, yeah, I guess you're right, it probably won't be worth $200,000 in 10 years.