Just because one stops needing to upgrade something, it does not make it obsolete. It often means the technology has simply matured.
That would have happened whether or not tablets and smartphones turned up on the scene. The speed of desktop computers reaching an adequate point for users has absolutely nothing to do with smartphones or tablets.
They are a separate technology.
Once it was suggested that laptops would replace pc’s. Didn’t happen. Then tablets. Hasn’t happened. Then smartphones - hasn’t happened.
These additional devices have not replaced others. They have become additional devices.
The argument that a technology becomes obsolete when it stops needing to be upgraded… or because other devices are being used is a weak one.
Put your desktop in the bin and try to make a living in IT. You won’t. And if you think that day fast approaches, then develop or focus your career entirely away from desktops…
99% of us won’t make that bet.