The "post-PC era" predicted by Steve Jobs at the launch of the iPad 2 in 2011 referred to the move from desktop/laptop to smartphone/tablet, and the research agency IDC has confirmed this trend; tablet shipments will exceed those of portable PCs this year and will exceed all PC shipments by 2015.
This must be a huge concern to Microsoft. All their traditional markets are receding, and they are struggling to get a foothold in the tablet market. The Surface RT has been a failure, and while there might be some demand for the Surface Pro, it's not nearly enough to offset the loss of sales in the laptop market.
I believe that Microsoft's marketing and product strategy might be flawed. Originally, it seemed like they might be on the right track. Commonise the look across their PCs, tablet and smartphones, and add touch to everything. It worked for Apple, right? Not quite. The requirements for work are very different to the requirements for personal. Like millions of people, I have a Windows PC at work that I am chained to for 8 hours a day. I despise that machine. After plugging away in Excel, it's a pleasure to switch to iOS for my personal needs. I seldom need Excel or Word in my personal life; Google docs will suffice in most cases. I don't want to have the same experience as on my work machine. Microsoft thought that people wanted familiarity, but in actual fact I think people want the opposite.
Microsoft are making the right move with Windows Blue, or 8.1, especially allowing users to boot to the desktop. My company will skip W8 altogether if it boots to a start screen full of social networking apps! 8.1 will bring W8 into enterprise, and I expect we'll see an increase in sales. Microsoft should concentrate on the enterprise market; a stable and secure OS and their dominant Office Suite. Their problem is that the desktop/laptop market has matured to a point where there is not a pressing need to update your system every couple of years anymore. In contrast, tablets are still evolving, and new technologies are helping manufacturers cut weight without sacrificing battery life. My 3rd generation iPad is awesome, but the iPad 5 will be very tempting!
However, I don't see tablets replacing desktops or laptops in enterprise anytime soon. People may use tablets for some tasks, but more for note taking, email, etc. Even if I could use Excel on my iPad, I would still need a PC to do the heavy lifting. For the foreseeable future there will still be a need for Windows PCs, and Microsoft may be losing some of the consumer market, and the replacement cycle in enterprise has increased, but there is still some life left for the PC.