All closed systems settle. Restrict a room of air flow, and you'll find all dust on the floor. Restrict a crowd from mind flow, and you'll find all persons in bonds with each other. You may not end up with perfect clones of each other's mindset, but Hubski is already quite leaning to certain sides of things.
Aiming to restrict oneself from outside opinions that might hurt one's worldview is poisonous, since there would be no one to question your behavior. In such an environment, thrive bitterness, anger and spite; nothing more. Take it from a guy who's spent his whole adolescence in self-isolation. This is not what we should aim at if we want to preserve what makes Hubski different from most other Internet forums.
It bothers me to see you being intentionally obtuse as you claim the following:
to argue all echo chambers are bad is to argue that we should never surround ourselves with those we agree with, but only constantly (exhaustingly) challenge ourselves with the company of those who hold firmly vastly different views
I suspect you would hold a grudge if I were to call you a social justice warrior while you were trying to fight for someone's rights, yet there's nothing wrong with doing so on its own. An echo chamber is prejudice confirmation ex nimia, not per se. Hubski doesn't see high tides of newcomers that often, and in such a community, fresh blood must be welcome. Those who don't accept the new things, stagnate. The irony of me refusing to accept the proposed new order of things is not lost on me. However, the tolerant can't tolerate intolerance, lest they desire to stand against their own principles.
(while researching Hubski's popularity on the web, I discovered that my nickname is right there on the screenshot of Hubski's main page from a while ago)
Point is: for a social creature, a closed system is a mind-killer. Take a gander at /r/The_Donald if you want to see what echo-chambering ad maximum looks like. Hubski's not likely to ever reach that point, given no etherial personage to get behind and worship, but it still experiences the effect of social entropy like any other community. The first step of preventing a condition is not aiming for environment that would allow it.