I think of GT3e as a first shot that shows what is on the way with iGPU. The biggest penalty it has is the huge die/cache setup. That's why I believe this is more 'proof of concept' along with low-volume MBP sales to sort of make a PR statement that shows that Intel can deliver great IGP, finally.
At the same time, if they had been given the ability to ditch the cache, and feed it 256-bit GDDR5 directly to the iGPU, performance would be even better. Sharing DDR3 main system memory is never ideal for good performance.
DDR4 256-pin SODIMM with Broadwell will probably be the nail in the coffin to anything but very high end dGPU for mobile. It remains to be seen if Intel is interested in pursuing this path on desktop, but with 4K coming on slowly and steadily, I would say it won't be long there either.
People shouldn't forget that dGPU isn't a static thing as well. By the time we see Broadwell w/DDR4, the 'Iris Pro GT4?' will probably dominate even the 750M by a large margin, but will be slotted up against improved future dGPUs.
AMD, otoh, is basically on the verge of losing all relevancy outside of desktop GPU. I'm not saying that to be mean, but just look at their current APUs. Even with the 6800K, you get pretty mediocre CPU performance, and the GPU performance isn't that hot either. Better than desktop Intel iGPU for sure, but the gap is getting thin, and will soon flip. Once you have lost :
Power efficiency
CPU performance per core
CPU performance overall
iGPU performance
iGPU perf/watt
Once all of that is lost, nothing to do but try to undercut on price, which is unsustainable in terms of bringing future competitive products to bear in order to attempt catching up.