Having Iris Pro only is not only possible, but logical in a lot of ways.
Going by the test results on Anandtech, the ECC memory in the Iris Pro makes it far better suited for compute tasks than the 650m. Granted, it would take a performance hit in gaming scenarios, but this isn't a gaming focused product.
It remains to be seen how it would stack up to a more modern discrete GPU, but Intel designed Crystalwell to be future proof for a while and I wouldn't be surprised if it remained better, or on par, with dGPUs in compute oriented tasks. Even without a dGPU in the refresh, Apple will still be able to claim some pretty lofty performance increase margins if they go by benchmarks oriented around professional related tasks.
Whether or not Apple does go that route remains to be seen, obviously, but if it lacks a dGPU I won't be upset as long as the laptop is better at doing what it was designed to do...
I think you have it backward.
Memory bandwidth is one of the main bottlenecks for Iris that Iris Pro "somewhat" remedies by integrating 128MB directly onto the CPU die.
But Iris Pro's advantage in computing performance over the 650M is because nVidia hasn't been up to snuff with software support for OpenCL. ALL of nVidia's GPUs (not just the 650M) suffer from bad OpenCL performance compared to competitors. This shouldn't matter much because not a lot of applications support OpenCL yet.
Move on to OpenGL (which is used in a lot of professional applications, like AutoCAD, Maya, Photoshop, etc...), and it's a different story. 650M completely demolishes Iris Pro in both performance and compatibility.
It doesn't take gaming to realize the performance benefits of a dedicated GPU in the rMBP.