Desktops are all but dead now...
Funny, I'm not experiencing any of this. Desktops are definitely a smaller chunk than they used to be, but they still make up at least 35% of the computers I see. People are using their computers less often, but they still need them more than ever. They just don't use them for stupid stuff anymore. Computers are no longer "fun" devices. They're devices for work. All fun is done on phones/tablets. Most of my clients try to use their computers for business or work purposes EXCLUSIVELY so they don't screw them up and have to bring them in for repair. But if and when their computers have a problem, they're immediately picking up the phone and giving me a call.
Though I still do get people that just use their computers for casual use. Just had a woman trade in a 2013 13" Macbook Air for an HP Elitebook 850 G1. The only thing she really uses the computer for is email, Google searches, and recipes. Her Macbook Air was absolutely FILTHY, covered in flour and other sticky crap because she keeps it in the kitchen most of the time. She was probably in her 30's. Good client. Dropped around $800 on the new computer and I got her Macbook Air for free in trade. There was nothing wrong with her Macboook. She just had a software issue, but decided to trade it in because she never really liked the Macbook. She uses PC's at her work and didn't really like Mac OS.
Another client I got that day just wanted a computer they could take with them between their main house and their vacation home. They already have a desktop at home, but wanted something they could easily take with them when they head up to their vacation house. They used it for email, internet, and typing documents. Not much else. Bought a $700 computer and traded in a 2 year old i7 Samsung laptop with a bad D/C jack. They didn't want to wait the 3-5 days it would take to get the jack shipped in, so they just decided to trade it in.
I also got in a 27" iMac from 2012 with a bad hard drive. I did software based data recovery on it and replaced the hard drive with a 1TB SSD. Cost them about $1,800. They only used it to look at the pictures they took with their iPhones and iPads, and for light internet browsing.
Then I got in a client with a gaming computer that a neighbor kid built for him with a bad graphics card. He corrupted his operating system by trying to reboot it dozens of times because he wasn't getting anything showing up on the screen. I also upsold him on a RAM upgrade, and a larger SSD. Total was around $1,000. My cost? Around $350.
This is all in one day at my shop, and these are just the clients that I PERSONALLY dealt with. My employees took care of probably a dozen more. People with money aren't giving up their computers. They don't see tablets and smartphones as replacement devices, but something that compliments the computer (or in the case of heavy smartphone/tablet users, the computer compliments the smartphones/tablets). You just have to market to the right type of people and you'll be swimming in high margin jobs before you know it.