> Ideally, a well-organized project would have a copyright assignment process to resolve those sorts of issues.

A lot of projects, notably including Linux, intentionally avoid copyright assignment to make it impossible for anyone to relicense the codebase. Making sure that there are thousands of copyright holders from hundreds of jurisdictions, many of who are not easily reachable or even knowable, all bound by common license terms protects the project from situations where some project participants would do something not agreed by the rest, either willingly or because they were forced to (eg. through bankruptcy).