People asking about graphics decisions in Apple's laptop line just need to think about 1 piece of information - TDP 15/28W for the Intel processors. The Radeon Pro 400 series in the 15" MBP has a TDP "less than 35W". People clamoring (like me initially) for dreams of a Pascal based nVidia 1060/1080 realize the GTX1060 mobile has a TDP of 75W....I would venture that is near or exceeds the TDP for the entire system. This is in line with the 900M series, but provides desktop GPU performance, but still......hot.
Apple did use the 750M at a TDP of 50W in the 2014 Retina 15" MBP, the the machines have gotten thinner since that time.