This is where the guys at AMD seem to disagree and I think they are right. Multi-chip GPUs in gaming need to be able to stand on their own feet. Developers are unlikely to put in the work to support that kind of solution (which I can understand considering the work involved and the possibility that the overall design may change in a 4 year timeframe). There is no indication that is going to change in the future.
Now, I would not dismiss the possibility of a multi die GPU with a central IO die and cache being able to work well in gaming workloads. The question is, can AMD make it cost efficent. This seems to be overall harder than on the CPU side.
We live in exciting times