As of now our crypto currencies are based off of the currencies we deal with depending on where we live.
If a given crypto currency is utilized in the market like grocery stores or everyday use such as buying a coffee every morning, how does the value of the item we purchase with crypto currency manifest?
Right now the crypto currency cost of an item manifests by it's counter value in the currency of the land where you are buying the item.

Example: coffee = $5.00; the crypto currency amount would depend on the current value of the crypto currency compared with the dollar.

How will a crypto currency set the value of that coffee if there is no longer a local currency to compare to?
What would set the crypto currency value of the coffee I purchase every morning?

What happens when all 21,000,000 BitCoins are mined and there is no more cash flow towards the system?
Will the BitCoin value freeze and it is no longer considered a fluctuating investment?

I am not saying that this will not work. I am just trying to wrap my head around this thought of how it will work once it takes over the hard currencies, if it does.