Let's presume that when 21 millions of bitcoin have been mined, Bitcoin price would be $100,000 each. So even 10bits (which is a very small amount of bitcoin 0.00001BTC) of Bitcoin would be worth $1 at that time, hence, the 21-million coins would be enough for everyone, but we are not going to be there soon, worry not.
In economics:
As the
demand increases the price increases and thus as the demand decreases the price decreases.
As the
supply increases the price decreases and thus as the supple decreases the price increases.
That's how the economics adapt with the demand and supply really. But still it is needed to increase the supple to make it balanced with regards of the demand because super high price results to a not so good scenarios though the users would really enjoy the high price of Bitcoin but it should not be so much, just enough.