Radicalization doesn’t just happen it has to make deep intuitive sense to that person based on their own prejudice. You can’t convince a normal person to kill people by showing them a bunch of videos but you can convince people not to believe hanlon’s razor and therefore deepen their hatred by false attribution of malice where none exists

Most content is "delivered" in the form of suggested videos from within youtube itself and other social graphs that seem unwilling to concede that they might be part of the problem itself.

The rate of spread is a problem of YouTube's own devising based on clicks, views and likes. Remove this gamification and you'll see less engagement, hence less scale of radicalization.

With youtube labeling people "radicals" is a deliberate polarising term designed to do little more than tar and feather people into a specific box. Just like segregation, just like Protestants, just like the German Jews... Etc. It's very unflattering.