Different jobs. A chief of staff exercises judgment; Heidi removes the drudgery that eats their week: chasing status, assembling briefings, answering repeat questions, keeping track of promises. Many teams want both; Heidi costs $300 a month.
The honest comparison is not Heidi versus a person, it is a person with Heidi versus a person without. The one with Heidi starts every day briefed, never re-assembles context, and delegates the follow-ups.
Where headcount is the constraint, Heidi buys back the hours that made the hire feel urgent, which is why founders are often her heaviest users.