The problem with this is 99% percentes, aka most average joes can't really risk their FIAT to buy stuff with BTC. We'll need bigger merchant adoption and more stability and more importantly better wallet and faster smoother behaviour of the software for it to reach a status where people make their daily expenses with BTC mostly. In fact even in the future it may still be seen as a legit store of value like gold rather than an actual currency. We'll see.
If consumers were to use their fiat to buy bitcoin that would then be used to spend at various merchants that accept bitcoin, then what is the difference? The answer is there is none. Consumers would essentially be spending the same amount of wealth/money, however the merchant would incur less costs in accepting the bitcoin so over time they would likely charge less for bitcoin paying customers.