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 think blockchain still possess another quality of data ownership? Open API would indeed facilitate interoperability as you mentioned but did not guarantee immutability. I might be wrong tho

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If the data is for me only yes, however if I need to prove a record to someone then not so much?

If it’s open you can just archive the data if you care about preserving it