At this point I really don't think CBS gives a **** about the ratings. It's not like taking big brother off the air is going to give them a better show to put in it's place, so if anything they would lose even MORE viewers.
Also, I think these HGs are going to be in for a shock if the house is split up and people are brought back. Right now they feel like they are so close to the end, but in reality they are so far away. Joe was still in the house last year at this time!