The "Apple tax" is becoming less and less of a factor as well. Apple products are generally perceived as expensive, premium gadgets that cost more than similar products lacking an Apple logo. At the same time, comparable smartphones and tablets cost essentially the same as their iPhone and iPad counterparts, and the deluge of ultrabooks designed to take on the popular Macbook Air are priced roughly the same as the Apple laptops.
The threat is certainly not imminent, though. Microsoft won't be going bankrupt any time soon. Mac OS X is nice, but many of the software applications that SMBs rely on simply don't exist for it...yet.
Large enterprises may purchase tens of thousands of PCs or Windows licenses at a time, but there are tens of thousands of SMBs out there for every massive enterprise. If enough SMBs and consumers jump ship to Mac OS X, it will also be a more viable market for developers, and for third-party peripheral manufacturers--which could attract more people to the platform. It's a self-feeding cycle.
Truth be told, Windows 8 is a great OS, and SMBs will appreciate many of the features and benefits once they get through the learning curve. However, Apple has an opportunity here to snag significant market share.