Repeat after me: "blockchain is only required if you want to solve a problem that deals with permissionless, trustless and distributed consensus".
- permissionless: no previous vetting/authentication/authorization of any participant. If you can a priori authenticate users, you can have a decentralized system with Paxos, and you can run a "decentralized" governance system based on simple "web of trust".
- trustless: no participant is assumed to be reliable/honest
- distributed: the network can suffer disconnections and partitions, but the overall system can still work.
If you can choose who will be allowed to define "consensus", you can use other BFT consensus solutions, so no blockchain is needed.
If you have a "cooperative" model, this assumes that you can trust the participants, so no blockchain is needed.
If your system can withstand network disconnections/partitions, no blockchain is needed.
Shouldn't that be "at least one participant is assumed to be unreliable/dishonest"?
"Innocent until proven guilty," the modus operandi of the US judicial system, at least in theory, is the design of a system that assumes trust and removes offenders.
When dealing with Byzantine-style problems, there is no such thing of "who is more truthful?", just "what is the consensus that should be agreed on?"
The private key must always be trusted for what the person behind it intends for it to do
Blockchain only requires people to coordinate on choosing the same programmatic criteria for block validity (and then choosing the longest valid chain).
One potential use maybe would be a blockchain that is openly run by a company (80+% of nodes or something) but if you also run a note it could continue after the company ceases to be.
There are other ways to do that but it might almost be a reasonable use case.