I'd argue that having such a bloated subforum makes people less likely to pay attention to the individual forums anyway

You pointed out yourself that it's a little redundant to have a whole subforum for only two forum members. While I'm not keen on treating subforum applications differently based on forum membership, I think that it may be necessary unless we start putting the subforums in slightly less visible places. Four parties already take up more room on the front page than the entire Assembly and every roleplay forum put together.

ETA: Plus, this doesn't enforce subforums being moved. It simply enables it. If this passes, admins can still keep the party forums top-level, but have the freedom to remove them from the top level if the bloat becomes too much.