When there are say 0.0000004 and 0.0000002 Bitcoins left who are you going to trade them with exactly ?
Do you read any of that. It is just numbers. You act like 0.0000004 BTC (0.4 uBTC) is a small amount. 0.4 uBTC (micro Bitcoin) is 400,000,000,000 aBTC (atto Bitcoins) or 400,000,000,000,000,000 yBTC (yoctoBitcoin). Look at all those zeros. Using 128 bit numbers for value it is possible to go way beyond that, 32 decimal places which means that same 0.4 uBTC can be used for 40,000,000,000,000,000,000,000 discrete, assignable, tradable units.
Who will I trade them with? How about every single human on the planet even assuming the population increases from 8 billion to 80 billion.Of course that even assumes the asininely improbable scenario where 20,999,999.99999996 BTC are lost will ever happen. BTW that is 99.999999999999809523809523809524% of the coins being lost). Somehow in 6,000 or so years we have managed NOT to lose 99.999999999999809523809523809524% of the available gold or 99.999999999999809523809523809524% of the available silver or 99.999999999999809523809523809524% of pretty much ANYTHING. Hell we haven't even burnt up 99.999999999999809523809523809524% of the available oil, coal, natural gas, or uranium. Unlike currency which merely trades hands being consumed (potentially an infinite number of times) those materials are are actually consumed in use and we still haven't "lost/used" that high of a percentage of it.
Still if it happens to Bitcoin ... it STILL isn't a problem. There may be a lot of potential issues with Bitcoin but running out is not one of them.