I guarantee you are wrong about GPUs.
There’s still tremendous room for advancement. We’re nowhere near the maximum capabilities of graphics systems.
Also, sound card advancement didn’t end when 16-bit cards came out. They have continued to evolve. However, they shrunk the hardware to level that is sustainable for onboard components. There’s no need for add-in sound cards anymore, just as there’s no need for add-in network cards anymore.
Discreet graphics cards aren’t going away any time soon. Hell, Intel just scooped up a dream-team to build it’s own GPUs.
There’s a case to be made for integrating GPU hardware into CPUs, as we’ve seen recently with the new Intel chips with AMD GPUs and the new AMD APUs with Ryzen graphics. However, those aren’t likely to overtake discreet add-in GPUs any time soon.
The big horsepower GPUs will remain independent cards for a long time still.
People who are satisfied with today’s graphics hardware are not the people that would buy the next gen hardware right away. But don’t confuse a few comments on a news post as a barometer for the market demand. There’s still a large community of gaming enthusiasts that will happily buy the best GPU money can buy, and there always will be.