The problem: suppose all global transactions are done using bitcoins. Then the value of a bitcoin should grow at the rate of GWP. This means that instead of investing in the stock market, I can just hold bitcoins and expect similar growth. I get the advantages of: liquidity, no fees, and maybe no taxes on gains. The more people do this, the less resources are invested in productive endeavors, and the lower GWP is. Unfortunately, it's in any individual's interest to hold bitcoins rather than invest them in an index fund. Maybe as a result GWP grows at 2% instead of 4%, and the world is worse off.
One might argue that the value of bitcoins would be bid up so that the expected rate of return would be closer to other extremely safe assets. Suppose at year t, each bitcoin is bid up to $X. Then at year t+1 bitcoins have gained less value than a share of GWP, as expected. The total value held in bitcoins has decreased in relation to GWP. But if rational people were OK with this new ratio of bitcoin price to GWP, why did they bid it up higher than this in the first place? And shouldn't that same reason apply at t+1, causing them to bid up the price again so that bitcoins do in fact grow in value at the rate of GWP?
NOTE: I am not claiming that individuals will not spend enough bitcoins. My argument is only about what happens to bitcoins that a person decides to save/invest.
