Is there a company out there that would publish a survey stating that individual buyers have actually said they chose a tablet to replace a PC? Until then we won't know for sure. Smart phones are so awesome these days their only limitations are screen sizes. They can do any regular office work. Movie production and engineering stuff isn't practical on phones yet. Soon the processing power will be good enough. Using Air-play with a tablet will allow even high-tech programs to work on phones and have the visual stuff be on a much larger screen.
Perhaps some manufacturers have commissioned such studies to learn exactly how many people are foregoing desktop and laptop computers for tablets and smart phones. Would they want to publish such data? Unless they are making tablets that are selling well my guess would be no.
How many iPads must Apple sell to equal the profit on one of their computers that wasn't sold because of a tablet? I for one will be buying a small tablet this year. If Apple makes a 7" model it will get my attention first. Apple has proven to me that they can give me a good software experience. If the price is in line with my expectations then Apple will get my money. If not, Google will get it. Toshiba makes a great large tablet. I'd go for it over an iPad if I wanted a big tablet.
What exactly will happen to Microsoft if Windows 8 is a failure? They still have the gaming devices and the office software. The OEMs still buy their stuff. Until the OEMs are willing to drop Microsoft the company will continue to do fine for years to come. They just won't be expanding anymore.