- It's a matter of thermal design, battery, point of failure (see all the problems with MBP's GPUs from 2010-11)
- dGPUs are not a must anymore, seeing how Iris 580 performs.
- If dGPUs weren't offered in the 13" when iGPUs were crapp (HD3000, HD4000...), they won't offer it now. I'd prefer to see increased SSD base storage instead, for the same price.
- You are not restricted anymore to having the same dGPU, now TB3 means eGPU with much more power (yeah, the ones supported, but I guess there won't be just 2-3 of them but many).
- There will probably be a dGPU option for all that want it: but it won't be a GTX1080m, it'll be an average dGPU like Apple used to use in MBP 15". And these average GPUs used will just perform slightly better than the iGPU. So I wouldn't consider it unless someone needs a bit more "mobile" GPU power. If someone needs raw power the eGPU is the way to go, and it will cost the same or even less than the plus paid for the dGPU option.