What do you mean when you say the "story for sharing data between processes isn't really solved"? My understanding of this is mostly academic because I don't personally mix languages all that often, but aren't there many established ways of communication between languages. You have IPC, unix sockets, FFI, protobufs, and other serialising/deserialisation formats. Not to mention the classic (but probably slightly overkill) move of spinning up a local HTTP server and using REST endpoints.

Yes, all those ways exist. I was more saying that if copying between structures processes using serialization is all you need, the JS ecosystem already has that.

The thing is if you need to do big expensive copies on the input or output it forces you towards a batch processing model, which is the opposite of incremental computation.