The ultimate advantage for bitcoin is governments can not stop convertability and it is relatively easy for individuals to hold and transact in bitcoin.
Do not doubt for a moment they won't try.
I can see the IRS being used to harass anybody who dares to withdraw actual bitcoins from these off chain trading platforms like Coinbase or Circle. Have you moved the coins since you withdrew them? That's a taxable event unless you can prove otherwise. Audits all around for everybody who asks for their coins.
If they can make it easier to keep your coins in an off chain system that allows for fractional reserve, and harass and punish anyone who tries to leave the system, they can play all the same games as they've successfully played with gold.
Accountability (via proof of liabilities and proof of bitcoin funds) is possible rather easily. No need to withdraw coins in order to force non-fratctional reserves. At least not in theory.
Bingo
To demonstrate your point one simply needs to ask how much of the FED's balance sheet is backed by real physical gold? i.e. How many ounces do they have vs. dollars in existence.
The answer is no one really knows. The amount of physical metal has not been audited in a very long time, and even if they did that it is not clear how much of that has been lent out as collateral to primary dealers (i.e. gold no one else has claims to). In addition to that the number of dollars in existence is difficult to say because the FED and US government is required to back stop so many entities, it is not clear how much "liquidity" they would have to create "on demand", i.e. what forced future money printing has already been promised.
Note: This ratio information was quite clear when the FED was first created and the country was on a gold standard. The ratio of gold-to-dollars fell and fell and fell over time, until it finally broke in the 70s and since then no one has known the ratio with any clarity.
In bitcoin, all of this would be transparent and clear. In addition if a central government went too far past a 1:1 ratio, you can be sure smart people would remove as many coins as possible from the fractional off-chain system, to directly controlled on-chain coins. This would cause the ratio to become worse and worse and crash the system in a final bank run, after which the exit window closes and no one trusts that fiat anymore. Fear of such a run will force central states to remain honest.