Look Bungie the rumble community isn't necessarily large, but it's dedicated they're always in there playing the mode. You're gonna ruin it for them the rest of the modes you can bunch up as you like, but rumble is the only "lone wolf" multiplayer mode. Are you really gonna lock that up behind a five week rotation? This can only hurt the crucible.
PS
If people really want Rumble to stick around then push the issue. Play rumble when you get on Bungie might take notice, and decide to change something, but i doubt it.
-
I love rumble. I'd rather it stays permanently as well.