Picking nits, but the decision to use plain text as the common data format in Unix does not rule out structured text (e.g. CSV, TSV, XML). Nor does it imply “human-readable.”

The original decision was about not proliferating specialized or proprietary binary formats, which was more of a norm back in the ‘70s than today. The goal was to make small single-purpose tools that communicated through a common interface (files) in a standard format (plain text). Unix succeeded and continues to succeed at that.

Nothing about those design decisions precludes tools using binary formats under Unix — image processing, for example. It just precludes using standard text-oriented tools on those formats.