I was thinking, you don't really need blockchains for composability/interoperability, you just need open APIs

permissionless composability, blockchains do provide

but if there were theoretically some non-blockchain tech that just exposed open data, and the end-result product that takes advantage of this data is the same, then the blockchain part would not be necessary for the end product

In this regard, money and markets are the real differentiator

It’s a fair point, money and markets require standards for contract and execution 

Blockchain isn’t required but some TCP / IP level standard combined with execution lends itself well to blockchain (mostly EVM with smart contracts)