That's mostly because you still need other currencies for day to day transactions (rent, utilities, most groceries etc). For BTC to not be compared to other currencies, you should be able to compare it to your life - that is, how many do you need to live the life you want. Atm you might be able to say "I need US$100 a day to live a comfortable life". If you can go a whole year (or something) and only earn and spend BTCs, you can start saying "I need 10 btc a day to live a comfortable life", and when everyone talks that way, your answer will be met.
That seems about right. My concern, however, is how will we get to that? I suppose the answer is that if more businesses accept BTC in easily accessible ways, then we can begin to lessen our dependency on fiat currency.
One more question though: do you see BTC ever reaching that point entirely? Will entire markets, companies, and individuals be able to forgo fiat currency completely, or will BTC always be merely a supplement to fiat money?
Yes the first step would be for lots of businesses to accept BTCs.
The second would be for businesses to start running on BTCs only. There has to be a distinct advantage to using bitcoins over cash for this to really happen.
The third part is legitimacy. Not many people want to invest everything they have into a business, only to have someone with more power (i.e., the govt) turn around and say "Sorry what you're doing is illegal, and we're taking everything you have". While bitcoins are not illegal, it's certainly got an underground stigma attached to it. I can pretty much guarantee you that if a government accepts tax payments in bitcoins, the rest will fall into place overnight. As long as you have to pay taxes in a particular place, you'll need fiat currency to achieve it. The alternative is for the government to accept bitcoins or labour in lieu of fiat currency tax.