Not entirely. They are already decoupled. The IGP simply advances at a faster pace with many more lower hanging fruits to pick.
New GPU uarch or improved uarch every 1-1½ years. A continually expanding node lead. 14nm vs 28nm. (We may even see 10nm vs 28nm)
Stacked memory will only have relatively minor effect on dGPUs. While it will frog leap IGPs.
And again, sooner or later the economic incentive to develop the dGPU further stops due to shrinking volume. Remember, the IGP doesnt have to beat the dGPU. Just destroy its ROI to win.
you're forgetting the situations where ROI isn't considered and functionality/performance dominates. Look at 8 socket platforms for instance. 99.99...% of people haven't worked with a current generation one let alone seen one in person in a non-demo environment, and yet they're still incredibly useful for certain tasks and if they weren't profitable, they wouldn't even be designed and produced.
most people don't find themselves doing tasks where money really isn't a major issue, but these situations do exist.
dGPU vs IGP, definitely not sooner. eventually, maybe when there's an easy way to offload work with a negligible amount of latency.
obviously, I'm not talking about consumers.