I would be very surprised if the Broadwell MBPs get a new screen or a major redesign. I do not anticipate any significant changes for the Broadwell MBPs that cannot be accomplished with a new motherboard.
There is price elasticity of demand for laptop computers. Apple can reduce their costs and lower the price by eliminating the discrete GPU. The cost reduction for Apple is so large that they can lower the price, increase their margins, and increase sales volumes -- if the Intel integrated graphics are good enough. Apple obviously judged Iris Pro to be good enough for the 2.0Ghz 15" Haswell MBP. With the promised improvements for Broadwell's iGPU, it would not be surprising if Apple were to consider it good enough for all MBPs. On top of all that, switching between iPGU and dGPU has never been flawless, Apple could use the motherboard real estate for other things, like perhaps more DRAM. So, the reasons for dropping the dGPU include cost, size, weight, and reliability.
I remember lots of moaning and wailing in these forums about how loads of customers would abandon Apple if they were to ever switch from SO-DIMMs to directly attached memory for the MBP. There was no mass exodus of customers and the same will be true when the dGPU is gone.
The path of progress in integrated circuits is ever increasing integration. I remember about 30 years ago similar discussions about how an iFPU would never match the performance of a dFPU. That was true then, as now, but missed the point. Even today, Intel could build a faster dFPU if they wanted to. However, by about 25 years ago there were no more dFPUs being made for PeeCees because iFPUs were good enough. By about 20 years ago, the high-end UNIX workstations were also iFPU-only. History is repeating itself with GPUs. Look at the level of integration in Apple's A7. That's the way forward.