Ok What he probably means is that the two hospitals are basically becoming one. So probably SG will end up closing, and some of the people on the show will lose the jobs. meaning that a spin off will happen, and Grey's anatomy will end with in the next two seasons.
The chief did this so that he could keep his job. You could tell by how he looked at Derick at the end. He didnt want to give up being chief to another person.
He did this to buy himself more time to save his job. I
I do not think that most of the main characters will leave, i think that Bailey might, but not right now. I think that when the chief saw torres at MW that he got an Idea while talking to the intern. Its all about the context clues.