Otherwise, if you're unable to source another script, you could just have another script return a normal result in addition to a JSON blob of the environment variable changes the caller should make, which is probably worse than just allowing sourcing.

Pipes allow arbitrarily complex networks of communicating sequential processes. In some cases, networks of tiny CSPs are cleaner, but without discipline, they can rapidly become worse than huge monoliths.

Particularly as programs start out simple and organically grow, once they start hitting length limits, they're going to start evolving into locally-distributed computation. Maybe you get something nice like composable small unix-like utilities. However, I suspect there's a large overlap between the set of developers who would do that well and the set of developers who would still keep things nice and modular within a single process if they didn't have size limits.