True, the speed of light is "absolute" and is called C, and also true is that the total amount of Bitcoins will be 21 million. Now, the sad thing is that the value/price of goods remains relative all the same, simply because these 21 million coins represent the total value of the Bitcoin-economy. This Bitcoin-economy can expand and shrink, and even certain sectors within it can expand or shrink.
Example. Climatical disasters cause a shortage of food, which make a bread more expensive; meaning one can buy less breads from one's income. Even in a Bitcoin stable mono-currency economy the price of one bread would increase. Hope this sort of helps in the way "we" need to think about money.