In my opinion, default offline vs default online is a HUGE architectural difference. You could probably try to hack it into the existing player, but it wouldn't work well.
A ground-up rewrite wouldn't surprise me at all for such a change.
A ground-up rewrite wouldn't surprise me at all for such a change.
I don't think it's a technical issue at all, but a "people pay for this feature wherever YouTube Red is available" issue.
From the screenshots, it looks to be trivial functionality that could be baked into the existing app. Besides the ability to download and play files is not something I consider to be a "HUGE" architectural change.