If bitcoin reach the 21 million coins limit, a lot of things would happen and the first thing that comes to my mind is that bitcoin will become a rare gem and the price will begin to increase very quickly.
When all 21 million bitcoins are mined, there will be no new bitcoin, only the already mined coins will be in circulation. Since no new coins will be produced, the existing coin will also be valued higher. But then the value of crypto doe snot just depend on total supply or circulating supply, it also depends on demand.
Bitcoin price is always affected by the law of market demand and supply. If Bitcoin mining successfully aced 21 million, there no longer be option for supply for Bitcoin which will push up on the demand for it. What we will have on our hands is Bitcoin scarcity. When that happens, Bitcoin price might go above $200k.