Good to see the mice are still winning the cat-and-mouse game. Selfishly, I kind of want the cat to start to win, to satisfy my curiosity. I predict that if YouTube ever actually blocked downloading, a YouTube competitor that supports downloading would start to immediately gain popularity. I want to know if I'm right about that, and there's no way to test unless Google actually starts to win. Go, Google, go! I believe in you!
I suspect that if youtube ever fully blocks video downloads you will start to see a lot of piracy groups and file sharing communities providing youtube content.
Those who download videos are a minority and targeting minorities will never give you exponential growth. Furthermore, the same minority probably abuses ad blockers so it would be difficult to squeeze a single cent from these freeloaders.