Basically, it will result in people not wanting to get better, and in turn not getting better because they find those mental health support forums a welcome community for them to participate in only as long as they have their mental illnesses.
What? Are you serious here? Are you seriously implying that mental health support forums actively make people regress when they're improving because the need to remain part of a community at the obvious cost of their mental health?
I'm not saying this doesn't occur, but I think you'd be more accurate if you were talking about forums that involved pretty much anything else. Games. Films. Clothing. Wicca. I can see this occurring in all these places but this surely must be frowned upon at the least in any semi-respectable forum devoted to mental health.
Or maybe I'm insane and Narcissism support forums are filled with selfies and Depression support forums rife with "Here's how to not improve anything and end it all" threads.
I kind of see what you mean with:
One other "group" aspect of social psychology is that people like people who are similar to them, and in a group they like those more similar to them at the expense of those different from them.
Which just makes the follow on even stranger. Why would someone suffering from X seek support from people with problem Y? That makes no sense. You talk to people also dealing with X. Y may be a related issue which they share but it's not the immediate and sensible go-to place.
Example - I have crippling stomach pains. So I'm going to get my eyes tested. By Mormons.
I'd strongly suggest that what you seem to be referring to are not good places to get help and advice. They are place you go to talk about how wonderful your shit smells and NOT actually deal with whatever issue it is you have.
If you've got some mysterious qualifications or knowledge, I'd disclose it now. More than a couple of us here are intimately familiar with mental health care, assessments, treatments, etc. and you seem to be talking out of your arse.