Wouldn’t you lose, for example, streaming capabilities once you use Celery? You would have to first upload the whole video, then enqueue the job, and then figure out a mechanism to send the thumbnails back to that client, while with FLAME you get a better user experience by streaming thumbnails as soon as the upload starts.

I believe the main point though is that background workers and FLAME are orthogonal concepts. You can use FLAME for autoscaling, you can use Celery for durability, and you could use Celery with FLAME to autoscale your background workers based on queue size. So being able to use these components individually will enable different patterns and use cases.