I don't think Bitcoin is ever going to replace USD or EUR. If anything, it's going to end up as an auxiliary currency like silver or gold. Rather than threatening governments, Bitcoin might actually help governments with their monetary policy once they start holding it as a "foreign reserve".
Don't be naive in thinking that bitcoin can be used to evade taxes. This might work with smaller amounts below 10,000 USD, but as soon as you start spending serious amounts of money (no matter whether 500 Euro notes or BTC) you inevitably leave a trail in meatspace.
I respectfully disagree with your point that bitcoin will not replace 'legacy' currencies. Because as of this moment, that is exactly what they are. There isn't an "if" but merely "when", in my mind.
As for taxes, you are correct as the system stands now - but when bitcoin replaces legacy systems, it would be more difficult unless governments adapted to such a system. One poster in another thread made a comment that if companies pay in bitcoin, it would naturally follow that governments would require some kind of control for tax purposes. That is entirely possible, but I'm not sure if that would be enough to save the legacy tax-collection system.
I suppose we'll see...