BTC's circulating supply is now 19 billion million out of the 21 billion million caps...
The only reason why the price of any commodity, including bitcoin, changes is determined by supply and demand. And if buyers refuse to buy bitcoin at the price of 70 thousand dollars, then sellers will reduce the price until this price satisfies the other side. Therefore, it is impossible to accurately predict the value of bitcoin without knowing the general state of the economy by the time BTC reaches Max Supply.
That is a good approach, not really thinking that you would be making a good profit no matter what is the key to this. There are too many people who are ending up with something like an idea of what bitcoin will do at certain point in time, but the reality is that they have no idea bitcoin would do, I am guessing that the best thing to do would be making sure that bitcoin could be anything by the time it reaches there and the general economy of the world will also matter into this.
I hope that we can realize this soon enough and not really go overboard with it too much. If we know what we are doing then it should be fine in the long term, just don't take too much of a risk and think you know it all.