Always thought the $100.000 was total bulshit but can't help hoping it will get there eventually.
Please put down the number you think it will reach not the number you hope it will be

Well next 4 years I don't think bitcoin reach $10k because now price is now $650 and its need long run to reach $10k.Still bitcoin can not reach to people everywhere in the world.First bitcoin need to introduce everywhere and encouraging people to use bitcoin when that's happening that time will be possible to reach the moon.
There will not be many people in the world use the bitcoin. If 1% of the world population use the bitcoin, the price will be over $500,000.
Already 1% people over the world are aware about bitcoin, the current price is $570 and the price expected by you is $500k, the difference is too large and if 30% people come to know about bitcoin then the price will touch $10k.
1% of the world population is about 70 million people. But just may 0. 5 million people actually own bitcoin.
we will probably never know how many people are actually holding bitcoin, but i am quite sure that the number is far below 1 million. maybe as you also said, closer to 500k. this however shows how much growth we can potentially expect in the future. keep holding and everyone will be just fine.
Its never possible to calculate the exact percentage of users using bitcoin, but one thing is sure the number is too low at a moment, hopefully it will increase in future.