The Hd 4000 can power a 4k display. It is not an issue of pixels.
The issue with framerates on the Retina was a software problem. Most likely because of the way Apple implemented their own scaling or the CPU limit in the largely single threaded web rendering.
Much slower GPUs have been powering much more displays and pixels for years now.
The 17" was an issue of price, demand and initially probably also yield on the new displays. They can throw them out now. Whether they actually do depends on demand. I think they don't like the low volume of such a product and that as tradition commands want to price it a significantly bove the 15" although it is only a couple dollars more expensive to make.
The speed of the GPU is definitely not the issue for why there isn't a 17". They cut the old because it would have competed with the 15" rMBP and a new 17" rMBP is probably not coming because of the low volume they expect with the strange pricing they employ.
With an extra thin bezel and a reasonable price markup, of just enough to cover the extra battery and display costs, it would probably be quite the seller. Big screen reasonable size and consumers can decide for themselves whether they want more mobile or bigger screen and not just have it a decision dictated by price for many.
It is not like Apple do that though, so I wouldn't keep my hopes up.