You're assuming that the new arrival has more information to offer than the departing one. I suspect the opposite is true. There's probably a sizable negative correlation between one's reluctance to hear uncomfortable ideas and the quality of the information one has to offer.
I think you missed the argument.
If you have a subculture or other group of people whose experience is strongly correlated with one another, and their conduct repels or silences anyone whose experience disagrees with theirs, then their view of the world will be missing a lot of information and will contain systematic biases.
We have words for this in various areas, such as "groupthink", "filter bubble", "circlejerk" ....
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.