Taking it to first principles, I don't see part time as feasible until documentation has negligible marginal cost. It takes time to get people up to speed.
Another blocker is the inane requirement in employment agreements by many large corps in the US that they own your IP while you work for them, regardless how it was developed or what the topic is (one place I worked at owned children's books if you published them, despite being a software developer). Until that is unacceptable, either legally or culturally, working part-time presents a conflict of interest. Rather than valuing the breadth a part-timer might bring, it's viewed as a liability.
Another blocker is the inane requirement in employment agreements by many large corps in the US that they own your IP while you work for them, regardless how it was developed or what the topic is (one place I worked at owned children's books if you published them, despite being a software developer). Until that is unacceptable, either legally or culturally, working part-time presents a conflict of interest. Rather than valuing the breadth a part-timer might bring, it's viewed as a liability.