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

i agree with this, but would add: a beautiful quality of blockchains is that they converge on protocol implementations that enforce 100% correctness of the data schema (other flavors of API fail at this often 😅)