Somewhere around 1998, a bought a PC desktop. It was grey tower, had some USB ports, a graphic card capable of doing 3D operations, a sound-card, and later I added an ethernet card. The new thing was the USB port, and an optical mouse. Today, at work, under my desk, there is a another desktop PC, the case is more black than grey, and it has more cores, more RAM, more disk space and a better graphic card, but it has basically the same set of features. The mouse is still connected to the USB port, and is still optical (and the laser is still red).
Basically, the desktop PC stopped evolving around 1998. The parts have kept improving and generally following Moore’s law, a few legacy components where dropped (the floppy drive), and various components have been consolidated in integrated chips on the motherboard (sound-card, network card), but those were all incremental changes, nothing ground-breaking. Strangely, they are still called desktop even though they invariably sit below the desk.
The box I bought 14 years ago was assembled by microspot, a company that still exists, but nowadays, they don’t assemble their PCs anymore. The box under my desk is a Dell, and even Dell is looking to move out of the PC sector.
When people talk about [desktop] PC dying, they don’t really mean die out, like the Dodo, they actually mean, become irrelevant, like mainframe computers. Nearly nobody talks about mainframe computers nowadays, still they have not disappeared: there are still many of them in various companies and administrations, churning around numbers.
IBM will happily sell you a z-series computer, and the POWER processor they sport are nothing to sneeze at, in fact the big irony is that while Apple switched from the POWER architecture to the x86 architecture in 2005, the same architecture has been powering all gaming consoles since (and one version of my NAS).
What will happen with desktop computers? I really don’t know. Maybe they will keep existing in their current form, satisfying a niche market. Maybe they will become more of a hobbyist thing, and include some Arduino like functionality, become more hackable. Maybe they will morph into something closer to NAS boxes.