Buying something with a currency, and paying taxes, are both other people buying the currency off of you. In one case they're buying your currency with goods and services, in the other they're buying it in exchange for protection. McDonalds gives you burgers because they want your USD. They want your USD because everyone wants their USD. That's how fiat works; the biggest difference between Bitcoin and USD is that Bitcoin is deflationary, while USD is managed by a central bank.
World currencies regularly collapse. What people fear might happen to BTC has already happened to the Zimbabwean dollar, the Argentine peso, and many others. It even happened to the denarius during the crisis of the third century. Even with stable currencies, there is plenty of risk to be had in FOREX trading. So... saying that BTC is a lot like every other fiat currency is not so much a complement as it is a neutral observation.