I think people forget that mobile OSes are minimal by design to aid ease of use on simplistic devices. Traditional OSes are indeed far from dead: The internet is still run by BSD ( All of the core network infra uses BSD ) most sites are served by BSD, Linux and derivatives. As long as servers are a thing, there isn't any end in site.
Operating systems will exist, the point being made is that which operating system most people use is irrelevant.
In the beginning you chose the operating system based on what software you needed to run. Now most people only need to choose an operating system that can run a decent web browser.
