>Perhaps in 20 or 30 years we will have something akin to plan9 or inferno composed from nodes on these networks?

Seems highly likely to occur within less than a decade. Tops. Just not on the blockchain. Likely something not-blockchain, e.g. https://ecsa.io/ (which /interacts/ with the blockchain, but isn't blockchain based.)

People massively underestimate just how fast technology progresses at the moment.

While the blockchain is clearly a distributed system, in a very real sense it is the antithesis of classical meanings of distributed computing in the sense that cryptocurrencies are designed to escalate their level of difficulty so as to fend off being accelerated by distributed processing, the exact opposite of most distributed systems that are designed to explicitly harness, or at least collaterally benefit from, parallelism.

What does it mean for an operating system to run on the blockchain?

Nothing. It is an impossible construct. The blockchain is a (very limited) distributed immutable storage system. It cannot run anything, and participating in a blockchain requires a minimal operating system to be present (storage/network/process management).

Operating System: - the software that supports a computer's basic functions, such as scheduling tasks, executing applications, and controlling peripherals.

Blockchain: - A decentralized and distributed digital ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the collusion of the network.

Is this supposed to answer my question?

No, just preface into their intended functions.

Edit: One might question what the ledger tracks in terms of operating toward user needs as opposed to financial activity.