Excellent write-up as usual! It is worth emphasising how easy it would be for a popular project like Gitea with fairly long history/logs to migrate forge instances. Gitea hosted Gitea(GitHub to gitea.com migration of the Gitea project) is WIP since February 2017. It probably wouldn’t take so long if a portable format like FFF exists.
Additionally, the simple file format allows for integration with third-party tools like Git and provide functionality beyond what could be predicted by the FFF library developers.
For instance, if a comment is edited trice, it is difficult to represent the same in REST API response without coming up with a method to represent history. If a forge implemented history in REST API, it would only serve to complicate the API and leak complexity into API-dependent projects. But with files, it is possible to offload history to a VCS like Git.
Another use case that the file format could enable is running analytics on the repository activity: the entire dataset is available for download in a simple and convenient format.