We can't tell if the price of Bitcoin will reach $1,000,000 in the next 10 or 20 years because it's something that can happen before the years you mentioned, particularly now that we have a lot of governments working on Bitcoin strategy reserve, which I believe will boost the level of Bitcoin demand.

Having said that, even if Bitcoin reaches the price of $1,000,000 per coin, if the total 21 million is yet to be mined, those who buy will still be happy they did, and the same thing goes for those that will buy after the 21 million coin is mined since the total supply is limited