"I was very interested to read that he expects the show to go into a S7."

I think Jensen's reasons are logical from a business point of view: can The CW afford to have two of its established shows (and two of their highest rated shows) both be canceled at the same time? He's right in that I could see The CW canceling Smallville after this season and holding onto SPN for one more season and use it to either lead into or follow a new show, to give the new show a chance to get a following of its own.