>Also, they can't "unlicense" previously distributed code

The United States allows authors (and heirs), except work-for-hires, to clawback copyright transfers and terminate licenses after 35 years on works made after 1977. This is an inalienable statutory right, which means it cannot be waived even with a contract.

That said, it may be difficult to terminate licenses in practice because open source licensing is done informally in most cases and courts haven't ruled whether this impacts the right of termination. However, copyright assignment and contributor license agreements are subject to termination.