The short answer
Free should contain everything a single person needs to succeed alone, and paid should start exactly where the value stops being personal and starts being organisational: more seats, shared workspaces, permissions, history, audit trails, integrations. That line works because it never asks a user to give up something they already love, it just charges when they want to bring the company along. Do not gate the aha moment, and do not gate on quality either, because a deliberately worse free product teaches people your paid product is also mediocre. Usage limits are the cleanest lever when the value really is per unit of work, but pick a limit tied to a number the customer already tracks so the upgrade feels like growth rather than a toll.