The only way smartphones will replace laptops (or desktops) is if they become powerful enough to replace them. Do children need a PC? No, but the vast number of working adults do, and will continue to need them in the foreseeable future.
I’d grant that mobile device usage is increasing at a great rate, but if anything is going to replace the desktop, it’ll be the laptop. Mabye in 20 years we’ll see mobile devices getting powerful enough to dock to a full size keyboard and monitor (make no mistake, people want and need a full size keyboard and monitor), and then to run spreadsheets, photoshop and other applications. But until then they’ll be an accessory.