Bitcoin is already scarce as it is. Once 21 million bitcoins are mined, no new bitcoins will be created, hence there will be fixed supply with infinite demand (the population keeps on increasing every year). In fact, the supply will keep on reducing due to lost bitcoins that can never be recovered. And no, it wont be difficult to buy bitcoin once all of the coins has been mined. There will be people willing to sell their coins at higher price. And then there are also altcoins that people might move to, in order to make profit.
The total supply is
20999999.9769 bitcoins, not 21M bitcoins. It's not big difference but the exact number is that, Bitcoin Wiki is used as a reference.
This question "What will happen with Bitcoin blockchain after about 21M bitcoins mined?" is basic interest of many people. It's good that they start with this question, because it shows that they pay enough attempt to learn about Bitcoin, and if they spend money to buy bitcoin, they actually have some good belief after having knowledge about Bitcoin. It's different than gamble.
Equivalent Network Time is a topic with insights about how more time will be required to mine the same 1 bitcoin, with different Bitcoin halvings and Bitcoin epoch.