I (mostly) earn in fiat, therefore I mostly spend in fiat.

It makes little sense to buy bitcoin, to only then turn around and spend that bitcoin -- at least, when I could have paid in fiat instead and wouldn't have needed to buy bitcoin.

Now if I were earning more in bitcoin, then the same concept applies. Why would I convert my BTC to fiat and spend in fiat, if I could just pay in bitcoin?

So, ... that's the chicken and egg issue. I won't spend in bitcoin until I earn in bitcoin. I likely won't be earning in bitcoin until my employer is receiving bitcoin revenue (with which to pay vendors and employees). And my employer won't be receiving bitcoin revenues until others then pay in bitcoin.

Someone needs to go first (by paying in bitcoin -- at least to those merchants who promise to keep those bitcoin circulating, in a bitcoin economy).