1) The cause of the PC's decline isn't tablets. The vast majority of tablet owners I know also own a PC. The cause of the decline is longevity. Gone are the days where you had to upgrade your PC at least every 3 years or it wouldn't be able to run the latest software. Today a 5-7 year old PC (core 2 duo) can run the latest software (minus games) just fine. If people upgrade half as frequently, the annual sales of PCs drops to half. Simple as that.
2) The rise of tablets caught him by surprise because most tech geeks have been in denial about the low-end consumption-only market. You first saw it with netbooks, which the technocrati constantly criticized, made fun of, and dismissed. Most of the reveled when netbook sales began to decline, citing it as proof that they were right all along (while ignoring the tablet sales which took their place).
News flash: Tech geeks comprise only about 5% of the population. The other 95% doesn't care how many GHz your CPU runs at, how many cores it has, nor how many FPS you can get in Skyrim. They just want something which lets them browse the web and read their email, maybe occasionally play a movie or some music. If you make something which appeals to that 95%, you only need to exert 1/20th the effort before your sales will match what you'd get selling something which appeals to the 5% who are tech geeks. Apple understands this perfectly, which is why they load up their products with frivolous "fashion" features that tech geeks dismiss. A glass back, a minimalist appearance, svelte tapering at the edges to make it look thinner. These mean nothing to the tech geek, but the masses absolutely love it. And they make gobs of money doing it because those masses are 95% of the population.