Yet the number of newly bought tower desktops is in free fall. Dedicated GPU is dead. Dedicated CPU is dead. For one it is dead today (ipad/iPhone only people), for others maybe in 2020, but SOC's are the only thing for the future. Moore is walking faster nowadays than we can think of new applications. Even a step from 1080p to 4K will only deliver a glitch in the strong line downwards. Turnover for the industry, size, heat, number of components, all go down very fast at the moment.
True, they've declined sharply. As have all desktop sales. In the case of PC desktops, it's not that surprising considering the huge numbers sold in bygone years. Ultimately, every market eventually peaks. Then it's usually a matter of course that sales decline.
As for this idea that dedicated GPUs & CPUs are "dead": IMO, there's no basis for assuming this. I agree that it may happen fairly soon with consumer Macs. Then again, the Mac userbase has always been relatively small. It still is. The choice of hardware has always been limited as it's wholly dependant on what Apple decides to offer. However, the limited range of available Macs doesn't reflect the computer industry at large.
Thankfully, things work very differently on the PC platform. There's always been so much more choice.

If Dell & HP stop making upgradable towers, it's really no big deal. Seriously. There are many other PC hardware makers that will be keen to step up to meet any demand for that segment of the market for years to come.
Keep in mind, there are hundreds of millions of PC users out there. Many still with upgradable towers that they've upgraded over the years. The idea that all of them, or even most of them, will suddenly have no need for upgradability because other computing devices are available, just lacks credibility.
For eg., I have a Mac Mini, a Mac laptop & an upgradable PC. All serve different needs. I may also buy a tablet soon. That doesn't mean I'll get rid of my PC. In fact, I'm very sure that, whether I buy a tablet or not, my current upgradable PC won't be my last one.