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Assume I have 10 bitcoins today. Ten years from now I go to spend them and have the 2% annual "fee" taken. I can only spend 8 bitcoins. I have lost twenty percent of the bitcoins I owned. That is a loss of value. If, as you stated, bitcoin is only a currency the exchange rate is irrelevant.
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Wrong! If the price of let's say a car in Bitcoins today is 10 BTC and the price of a comparable class car in ten years is 8 BTC you haven't lost any value. You have lost a
nominal amount of 2 BTC, but have at the same time retained the full value of your assets in BTC.