Sorry but I have to say it "Betteridge's law of headlines" so NO. If by mobile first you mean smartphones and tablets, absolutely not, these don't have enough

Sorry but I have to say it "Betteridge's law of headlines" so NO. If by mobile first you mean smartphones and tablets, absolutely not, these don't have enough processing power to replace desktops unless all you do is email some docs and web, anything intensive (IDE, virtualization, image/video editing, vector design, 3d, and the list goes on) will require at least a laptop. Also most PC gamers will keep on going with the desktops not even laptops (I'm more a casual gamer now but the games I run don't even exist for any tablet, and most laptops will have to run them in low settings)