My opinion is that discrete GPUs will always matter , for a single reason : consoles .
Consoles will drive graphics sophistication forward , requiring powerful discrete GPUs to match them and to exceed them for the purpose of leveraging the PC (which GPU makers will make sure it happens to keep their business alive).
Not to mention that consoles will depend on the advancements made in the sector of discrete GPUs to build their graphics sub-system , just like what happened in this console cycle.
Well, im not sure this generation of console is a good example, but in reality even if they are APU based ( Like some rumors suggnes ), console are different and they are "based on" APU, its not a simple APU..
you will need a bit more power and so effectively discrete gpu for match the simplicity of they work they have to do ( no need to run full OS and instructions, they run with specific API and optimisation )
I agree with you, discrete cards will still exist, and not only on professional space.. Computing take more and more importance, even on games and lets be honest, it will be really hard to match the performance spot with just APU.
Replacing our CPU + discrete gpu by SOC for gaming ? i let all try the new 3Dmark and watch the test who will be ported compatible with tablet and smartphone for have an idea and compare it with the last benchmark of the suite, Fire Strike.
Even if its not fully really representative, you see where the problem lie. Why peoples will go back on power and performance ?
I was compare the performance of my old gpu's after finding some screenshots of score and benchmark on a back up. Wow.. even thoses last 5 years, the discrete gpu graphics have got a massive amount of performance gain and on all domain. Lets not forget the average users have goes from 19-21" to 24" monitors screen thoses last years, this have a cost in term of gpu performance and looking the trend of the last 10years, this will continue to increase.
Its clear, games after games, we maybe not see a massive increase on quality unless some case, maybe its sometimes due to the lack of art of some developpers or they just dont implement graphics / effects features correctly when they port games to PC. ( massive drop on fps for a limited visual quality enhancement ) or sometimes the change is just " subtil " ( shadows, light effects, or particule is a good example, if you dont play with the max setting, you will not even see a real difference untill someone point it to you ). But play some games who was made 5years ago, and compare them of what is made todays.. ( like an old Hitman vs the new one ), and you will clearly see a massive difference. And more you watch it in details and more you will see the difference.