So, even in a perfect world where BTC is the only currency, don't you think there should be a way to let everyone pay their taxes? Then HOW, that's the tricky part! Directly - you pay depending on what you earn - this could be done by your employer OR indirectly -you pay when you consume - like VAT? Dunno... 
Why do taxes have to depend on what you earn? Why not just have a simple system where everybody owes the same amount of money every year, wouldn't that be more fair?
The how of paying taxes with bitcoins is really easy: the government sends each citizen a bitcoin payment address, they send the bitcoins owed to that address.
You are sort of missing the big picture here. Bitcoin is transformative in the realm of public works expenses.
Bitcoin would be MUCH better for taxes/community purchasing. A community could very easily use highly granular micropayments for services used, and provide greater transparency for costs and payments for those.
It could entirely obsolete the very stupid broad-brush taxation schemes currently used like VAT, Income tax, inheritance tax and the rest by making fee for service more pervasive. These census based headcount taxations are thousands of years old and very obsolete with the diverse tapestry of humanity today.
It would also allow for better transparency into the government accounting if we forced non-anonymity on the government money uses and required our governments to use cryptocurrencies.
We are at the very early days yet, but the potentials are truly amazing.