I think you are spot on with this. I never understood why all blockchains have currencies. Especially most of these altcoins.
Do you think it's possible that blockchain technology was created by an AI? And the cryptocurrency part was just put there to give us humans an incentive to build the infrastructure through building mining rigs.
Think about it this way: AI needs physical computers to exist, and as humans mine crypto thereby building the blockchain, they continuously expand the world wide AI brain! The humans have a financial incentive to build the computers.
All three of your reasons as to why blockchains have a native currency fit into this idea perfectly. What do you think?
RE: Could we be wrong about cryptocurrency? Blockchain without a currency!