People buy bitcoin for different reasons but the majority do it to gain profit and the common one is buy dip and sell the news, mostly not when there is ATH but when they have had enough profit. That's what they do continuesly. The reason why this is also common is that Bitcoin is highly volatile and in every of his cycle, it will go up and down, these continues waves is what traders utilize to make their profits.
We have some countries that has excepted Bitcoin as a legal tender, El Salvador is one of them and as I'm writing this, he is been re-elected for second time president of the country where Bitcoin is free to use for all, you are free to use bitcoin to buy what you want over there. We also have some companies that accept Bitcoin too. As for the 21 millions, the rest of the supply will not be fully mine until the next 100 years, so that's not a problem.