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.
This is true. We, as Bitcoin users, got a long way to go before all the Bitcoins will be mined. Would be nice to teach our next generation about Bitcoin and pass it over to them.