The examples in the post are all relatively recent, but if you look at the retention stats for these projects, OCD in particular had the highest. The stats show the long term outcomes.

How do you encourage people to mentor and/or engage with new users? They need incentives. If you do provide an incentive system, that means being trusted as they are being provided with resources.

You are right that engagement is a major factor in retention as well, but how do you maintain engagement when the platform is bleeding users and by extension activity? You need to address the lack of incentive to bring people in if you are to resolve lack of engagement.