I wouldn’t call desktop systems and OSes a “dying business”, just a mature, commoditized one. I work on big databases on my home desktop system, and even for general browsing, the desktop computer is scads better for anything that isn’t inherently mobile (e.g. GPS) or needs a touchscreen.
Besides that, businesses live on desktops and laptops (which are often used as desktops. Mine at work is pluigged into three monitors, and a keyboard designed for grown American man hands). Microsoft still makes lots of money and lots of profit, and will do so as long as they can hold ontothe enterprise.
Cell phones are getting to the point where they have enough computing power where they can do all that. You would only need a dockjng station to plug in your phone that everything else plugs into.