The divisibility or the denomination of Bitcoin solves the problem; value can be proportional to the fraction of Bitcoins that you own.
The value of Bitcoins divided by the population will definitely be much lower, due to the burned coins, either intentionally or unintentionally. Bitcoin is designed with a decreasing inflationary trend, but by the definition of purchasing power, then we've had quite a bit of inflation in the recent years. Bitcoin isn't affected by possible inflationary problems; because it is infinitely divisible. However, Bitcoin is an entirely new concept, or an experiment of a currency where there are no external mechanisms to regulate the economy. Whether this is feasible in the long term, remains to be seen.
Some currencies like Monero uses a tail emission system, with a constant inflation. This means the block rewards are constant after some point in time; perhaps more than sufficient to complement the lost coins and still provide incentives for the miner. As I've said, the stability of the network after the 2140 deadline (probably far earlier) remains to be seen. Economics theory holds only with ceteris paribus, but obviously that is never the case.
For all of you that thought Satoshi actually analysed the economic impact of that number:
My choice for the number of coins and distribution schedule was an educated guess. It was a difficult choice, because once the network is going it's locked in and we're stuck with it. I wanted to pick something that would make prices similar to existing currencies, but without knowing the future, that's very hard. I ended up picking something in the middle. If Bitcoin remains a small niche, it'll be worth less per unit than existing currencies. If you imagine it being used for some fraction of world commerce, then there's only going to be 21 million coins for the whole world, so it would be worth much more per unit. Values are 64-bit integers with 8 decimal places, so 1 coin is represented internally as 100000000. There's plenty of granularity if typical prices become small. For example, if 0.001 is worth 1 Euro, then it might be easier to change where the decimal point is displayed, so if you had 1 Bitcoin it's now displayed as 1000, and 0.001 is displayed as 1.