Possible, but unlikely. You can still buy the 6, 6+, 5S, 5C (although that seems to be in the process of being discontinued) and the 4S, as well as the 6S, 6S+ and 5SE. Apple usually keep a few older generations in production to satisfy lower price points.
The only time I can think when this didn't happen was when the iPhone 5 was replaced with the 5S and the 5C. This was because the 5 had only very primitive 4G support, and wasn't really a viable product to continue with (at least, from network operators' point of view). Well, I've never read that reported anywhere, but as someone who follows the industry it seems very obvious to me.
Anyway, I can't think of any reason why Apple would discontinue the 6S and 6S+ when they launch the 7, but of course we will have to wait and see.