I wouldn't buy an APU and a discrete GPU system.
The i7-5775C with its Iris Pro Graphics 6200 and the Ryzen 5 2400G with its Radeon Vega 11 Graphics
are APUs and their graphics performance is decent enough to make people want this additional acceleration, for example on top of their usual RX 550, RX 560, RX 570, RX 580 and GTX 1050, GTX 1060 discrete graphics.
This acceleration in itself can add frame rates to make the difference between unplayable and playable experience in a given scenario.
Why would I want my 4770ks IGP to work with my 290x?
Because you want more frames per second and you want to improve your gaming experience.
It would be amazing.
It is in AMD's interest to drop the monolithic, difficult to design and manufacture large chips, and instead should focus on small, relatively cheap to design and manufacture ones.
In this way they will be sure that the scalability returns, and isn't limited by a wrong architecture.
Imagine next-gen graphics cards similar to Ryzen.
The ultra-enthusiast cards to get 4 small chips, acting as one large.
The performance segment cards to get 3 small chips.
Mainstream segment cards to get 2 small chips.
Entry segment cards to get 1 small chip.
Some people say multi-GPU future will happen when a solid wall in the performance of single GPUs will be reached.
The thing is that even today graphics card solutions lag behind the available display resolutions.
But the majority of graphics cards make 4K gaming quite unpleasant, not to mention 8K that will need 4 times as more power. Where will it come from?