Why does it have to be decentralised (by which I assume you mean permissionlesss to join as a validator?)

The only reason for this - it would seem to me - is the ability to have nobody in control who can be subject to law enforcement.

If you need this kind of decentralisation blockchain, and all its inefficiency, is the only choice.

Societies should not require such things though. They need to have trustable institutions and intermediaries to function, in finance and many other areas.

> Societies should not require such things though. They need to have trustable institutions and intermediaries to function, in finance and many other areas.

...which is more or less the same conclusion that I've arrived at by the end.