I think there is a space for private (or permissioned) blockchains and there are a few techniques that allow such a chain to inherit the trust of a public block

I think there is a space for private (or permissioned) blockchains and there are a few techniques that allow such a chain to inherit the trust of a public blockchain such as Bitcoin or Ethereum (side-chains, Plasma, etc) so that it's possible to use cryptocurrencies or other popular tokenised assets in a private setting without users having to trust the owner of the private chain itself. In such scenarios, the role of block producer is centralised and limited to the owner of the private chain which means a less power-hungry consensus mechanism such as Proof of Authority can be used and have block interval times in the milliseconds. A project I've worked on recently aimed at delivering a similar solution in a global company that has historically grown through acquisitions and mergers where the IT systems never fully integrated, meaning that despite operating under the same name, there are hundreds of BUs that don't "trust" each other.