PCs will never become obsolete.
Tablets will become more popular among those who only use computers for social networking and email/messaging. laptops will be more popular for those constantly on the move and travel a lot, but PCs will always be the power houses with the most flexibility.
I have a smart phone, tablet, laptop, and a PC. I spend the most time on my PC, but when I travel for work I take my laptop and tablet with me. Laptop for the hotel room and tablet when I'm just out and about.