Right, but you're also operating under the assumption that the 13" model will be refreshed any time soon (nobody but Apple really knows the answer to that). I doubt Apple will have any reason to do so until next year - the current model is less than 2 months old.
I suppose if you don't mind waiting, that's fine. I picked up my 2015 rMBP because well, I needed a new laptop to replace my beaten up old 2011 model - and I couldn't afford to wait.
Battery life for me has been pretty stellar on this one. I get 9-10 hours on Safari web surf, which is about in line with what Apple rates it for. For a notebook with a high-res display and class-leading CPU, that's pretty impressive already, at least to me.