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
Where supply is limited but demand is unlimited, it is natural for the price of anything to increase. The situation is similar for Bitcoin. With a fixed supply of 21 million, demand for Bitcoin is increasing steadily. Those who are able to hold Bitcoin patiently will definitely benefit. I think Bitcoin is possible to reach 1 million after the next few halvings. Moreover, if it happens before that time, there will be nothing surprising. If governments start reserving Bitcoin then it will not take long time. It can reach 1 million within a few years. There is no way to predict the Bitcoin price with certainty, but if someone continues to make their investments regularly depending on the possibility of the future, they will definitely be able to benefit the most.