He predicted and theorized free software 10 years before it happened in Tools for Conviviality, made the most obvious and needed critic of education and hospitals alone against the Zeitgeist, studied step by step a lot of field of society to find patterns to simplify understanding.
He created simple concepts that everyone should know —- counter productivity, vernacular, iaotrogenic, radical monopoly, conviviality, poverty vs. misery etc.
He is much more pragmatic than all his leftists colleagues. He might not go very deep in economics but at least he’s not a basic marxist. He might not go as deep as Jacques Ellul in his critics of technology, but at least he is very understandable, anyone can be inspired by his books. I read most of Illich writings at 19 years old and it stayed with me for years