Two questions. One simple and one perhaps hard.

What happens when the total supply is reached? What is the incentive for mining after all BTC's are mined?

Supply: It is often said that BTC is valuable because it is scarce, citing the 21 m total BTC's. But each BTC can be subdivided to 1/10^6 which makes the total supply 10^6 larger. Whether one BTC equals to 1$ or 100$ or 0.00000001 BTC equals to 1$ - what's the difference? The total quantity of Satoshi's is 21 * 10^6 * 10^8 = 2.1 * 10^(15).