As long as there is a need for powerful computers, there will be desktop computers.
I remember 20-25 years ago, companies like Novell were prophesying the end of the desktop computer, and applications would be run on servers with the user just having a monitor.
Funny, because that existed 20 years earlier and dumb terminals went the way of the dodo. I remember 23 years ago working in a computer company and the first Toshiba laptop with an active matrix screen came in - and it was said that soon, everyone would just use laptops and there would be no more desktops.
Bottom line is that the death of the desktop has been predicted for at least 25-30 years, and it hasn't happened and shows no signs of happening. Who is going to be doing PCB design on a tablet or smartphone? There will always be aspects of a powerful machine that are incompatible with a small machine. Such as screen size, storage, power consumption. For as long as this universe obeys Newtonian physics, more speed will mean more power which will mean more electricity, more heat sinking, more physical size, more expense, etc. Some people will pay a premium for that power (gamers, video editors, CAD users, software developers). And those machines are where real development will happen.
Could Linux take over as the OS? I guess so... but that's been supposedly coming for decades as well and also hasn't happened. Windows is dirt cheap, supported by a major company that you can call for tech support, and has the largest user base and largest software list out there. What is the incentive to change? Security? Windows is really good these days. Cost? A total non-issue. Sticking it to evil Bill Gates? Corporations don't care - they just want to make $$.