I download for offline viewing quite often. It is available only on mobile devices and the videos are available offline for 30 days post download. Solutions exist to view video from phones on laptops.

But all that doesn't go to my argument which is that YouTube, not myself, may not see your use case as legitimate since they provide a subscription service which enables offline viewing. I doubt that because the way they have implemented it, for whatever technical, contractual or just on whim reasons, may not meet all your specific needs they see you bypassing controls to implement in your own manner as legitimate.