does that mean that people would not be able to buy it again?
Why wouldn't they be able to buy? Even if every single bitcoin gets mined today, people will still be able to buy. Why? Because the markets are open 24/7 and there will always be sellers in the market regardless of what the current supply is and what the future supply will be. The chances of there being absolutely ZERO sellers is next to impossible.
Yup, think a reality on how if all over the world if the gold was already mined? So if this was happen then the gold can increase the price depend on the demand of it. It is like crypto when the supply limit then the demand would increase, the bitcoin will be more expensive.