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 $1 at that time, hence, the 21-million coins would be enough for everyone, but we are not going to be there soon, worry not.
Bitcoin won't get to 100k$ if Lightning Network doesn't get added and from what i see, we will never have Lightning Network and Segwit because of Ver&Wu
When bitcoin gets to 100k$, you won't be able to make small transactions because the miner fees will become extremely expensive. If the fees become expensive, people will stop using bitcoins. If they stop using bitcoin, it's price won't rise. So bitcoin price depends on how big its user base is.