Decentralized, trustless systems are good because they create a situation in which malicious behavior causes losses, while following the rules makes a profit. Thus, the system remains protected even if each of its participants ready to become malicious.
A witness has to have reputation and be known. It is a matter of image. And government/s has an image too. It has balance.
Your model say "Government won't do any harm because i believe they won't do any harm".
In a crypto community, it is customary to trust systems based on economic initiatives and not on hopes.
Imagine situation. Satoshi, Ross Ulbricht, Mark Karpeles, Roger Ver and 8 other well known people are witnesses of Bitcoin. FBI arrest Ross, shut down SilkRoad. Do you really belive rest of witnesses will not be arrested?
If each witness node (no person) does their job, they receive aprox 1/12 of the half of fees I post. They need bytes to post each 10 minutes or so. So it will be sustainable because simply users use the system.
This means that even with the node operator in jail, system still working because witnesses operators will have put their node in a safe location, out from the hands of anyone and routed by TOR, like Rogier did.
If witnesses are arrested due to being byteball witnesses, I can guarantee you, world would have become something that you would desire to be in the world of the 1984 novel.