Once you have a central authority that everyone trusts, what does the blockchain add?
As long as mining is involved, I don't agree with that argument.
With central servers (and today's banking system), the government can freeze bank account. Even if today's laws would prohibit that, there is no guarantee that this will also be the case in the future. Thus, the only way to plausibly guarantee that the government won't freeze your digital assets, is by issuing them on a decentralized infrastructure under the control of the public (e.g. as colored coins on the Bitcoin blockchain).
Once nobody can tumble legitimately then quite literally only the criminals will tumble and there you go.
Really, you can dictate whatever you want as this central authority, but enforcing it is a whole different matter.
That could never happen in a free country.
Also there's nothing stopping someone from evaporating the coins via mining fees. So you'd have to have dictatorial control over all miners as well.
...Which we most definitely do not have.