Nobody subscribes to every Youtube channel.
So what is your point?
It's a model that work and scales to half a billion users per month.
Reddit's moderators are volunteers with iron fists who only police tiny sections of reddit. Admins get involved in TOS violations, not direct moderation.
Your comparison makes no sense.
What moderation? For what are you talking about? From what I gather they predominantly rely on automatic filtering, with barely any human filtering. They don't even moderate some of their largest channels.
E.g.
LinusTechTips complaining about spam junk and community fed solutions. https://www.youtube.com/watch?v=zo_uoFI1WXM
MarquesBrownLee doing the same. https://www.youtube.com/watch?v=1Cw-vODp-8Y
Reddit does the same broad stroke in that yea they have broad generic spam/bot detection, and also rely on reporting moderation and community driven tool. They already operate in a similar/better model than youtube.
The original argument was that Reddit/HN couldn't handle 5% of Youtube community and I am calling nonsense to that.
That's false, admins do get involved in direct moderation, and Reddit has a whole team of admins (Reddit employees).
Nobody subscribes to all of Reddit.