This is now implemented. Main chat rooms do not freeze automatically due to inactivity.
It looks that the above is wrong (see the first answer to this Q and newest comments to the above referred answer).
Another finding from Meta SE, Main chatroom for a SE site and chatroom pruning,
The SE software keeps always one room for each SE site alive. But no particular room is chosen as the "main" room for a site.
By the other hand, what is the point to have a chat room that is use very rarely and the most common topic over the last months is
Posting a dummy post to keep the room from getting frozen.
dummy post returns 22 posts.
When someone post a comment, a widget is shown on Meta right panel
It's a bit annoying to find that the newest message in the last 2 weeks is a dummy post. Considering that to create a new chat room is required a rep 100, and that to participate on Meta it's just a rep 5 and usually people that require a someone to talk gets and answer faster on Meta than on chat...
Please, stop posting dummy comments.