Risk. The decreasing yield of mining ensures that the future value of bitcoin will appreciate (assuming demand stays flat). There is no risk that supply might suddenly increase.
In that scenario, how is spending Bitcoin any different, micro-economically, than foregoing the opportunity to invest in a similarly-appreciating mutual fund? What's absent from my model?
Assuming people are rational actors, they would spend as little as possible and hoard the rest.
So then we need to ask: "how is holding bitcoin different than investing in a similar mutual fund".
Holding bitcoin decreases the money supply driving up value artificially, whereas investing in a mutual fund increases the money supply available for investment and drives economic growth.