I'm sure people used to say the same thing about the dollar or bank notes, and exiting to the underlying asset like gold. It is possible to remain within the Bitcoin system. It may be difficult now, but it's getting easier every day.
As long as the government is collecting taxes, and as long as you cannot make tax payments with Bitcoin, you cannot remain within the Bitcoin system. Even if all your other transactions are done with Bitcoin, you'll eventually need to get some dollars to pay your taxes -- and then you'll be hit with regulations.
As for gold versus paper money, the above reasoning does not apply. Paper money is accepted for tax purposes (and would be far less popular if it was not).
