If you look at TFLOPS, the 580 is more powerful than the current dGPU.It's not like the dGPU can't get thinner or less power-hungry. Tech advances. Also, define 'significantly thinner' because it's probably going to be pretty minor compared to the current model. The Iris Pro 580 is an improvement, but it will pale in comparison to what a current dGPU can do (even a fairly weak part like what AMD is offering up).
Anyways. I'll bet my account that the 15" has an option for a dGPU and that it's a Polaris part.
It's around 15-25% thinner (probably 15-20) - however even lower than that would significant when it comes to internal components.
You're right, tech does advance and Polaris is much better when it comes to power since it's 14nm. But i'm not the one you need to convince, I think they should include a dGPU on all MacBook Pro models (or at least on option for both 13" and 15") and I would be willing to sacrifice an hour or two of battery life to have that.
I am not saying for certain that they'll be dropping a dGPU model, but it is likely to happen
I really hope they don't drop it - I don't care about the downsides of ports, the keyboard, and Magic Toolbar, graphics is the main concern for me.