If bitcoin becomes large enough, governments will simply say "Anything you buy with bitcoin will be taxed the same way you did with dollars. Oh by the way, you also have to show us how you earned that bitcoin, in the same way with dollars."

Eventually, I believe one of the two things would happen:

(1) Bitcoin becomes part of the "system", like Paypal, Amazon, and Visa. It remains one of the handful of convenient methods of transaction for some situations. Some of its early adopters publicly denounce it as corrupted by the government and mega-corporations, and move on to greener pasture.

(2) Bitcoin loses its initial lure and dies off.