"Nobody even really sells desktop PCs anymore except custom gaming rigs and low-power kiosk boxes."
I believe you should embrace the novelty concept called 'work' sometime. Then you may see some 'niche' application of PCs.
Mobile devices can, yes, be sold in bigger numbers, may produce higher profit, many trivial applications like browsing for news and sharing our newest and greatest experiences by tweets and photos do not require, luckily, a desktop anymore and so these paramount usages could been shifted to the only important platform of mobile devices, yet the second grade activities of design, manufacturing, academia and so still rely on the archaic concept of desktop computers and they keep alive this dying artefact. Even traitor mobile developers (all of them, the fools!) dare to use desktop computers still instead of solely relying on mobile phones. But not long, soon the last aircraft engineer or corporate accountant will sell their desks and throw away the last of the keyboards moving to the only necessary platform of mobiles so the PC can go extinct finally!
