Mobile devices are becoming more mainstream, but they still can’t compete with pcs on their power/life ratios. As mobile devices become stronger, batteries get drained faster.
I have a Galaxy s3 and I love it. I’m a ui designer and app developer, I’m also a gamer. Those are three activities I do not want to do on a mobile device (except maybe very casual gaming). Laptops can match some of that power, but the cooling solutions are often not as good as what you can put in a PC.
Smartphones and tablets are booming because they are very casual friendly to the public and good for entertainment, but word processing, 3d modeling/rendering, animation, video editing/rendering, design, programming, and heavy gaming need the power of good cpus/gpus and with no interruption in battery life.
I think that until there is a mobile device with the power to match PCs (but not drain the battery), PCs will be here for a long time.
In terms of building web apps, that’s why responsive design is so big now. To address the variety of devices connecting to the internet. Mobile devices and stationary devices have always survived different purposes, and perhaps always will. You have to have a mobile phone anyway, so smartphones are a great investment. Tablets on the other hand are more niche. But you wouldn’t replace your computer with your phone, and for some of those same reasons, you wouldn’t replace your computer with your smartphone (maybe for the least complicated tasks).