I'm not saying anything about ideas ceasing to exist. Only that ideas discussed in closed communities which have become echo chambers are never challenged. Members of those communities will never hear anything but the rhetoric of the group they are isolated to.

This is just as true of good ideas as it is of bad ones, but that's far less of a problem because groups of people spreading good ideas among themselves isn't typically dangerous, although there are a lot of advantages to having diverse perspectives in every group.