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Why would you not want to consider the step failed if it didn't produce all its expected targets?

Yeah, that's my point: if you're happy with this all-or-nothing logic, you can just use dummy target files.

Allowing steps to partially succeed seems like adding quite a lot of complexity, and I'm not sure where it would be beneficial.

Dummy target files don't work if the files are deleted elsewhere --- i.e. not as part of the make pipeline.