If you are in a big business, you probably do not mind paying another $1,000 for some fancy OS with bundled support. But if your resources are low, you will look for cheaper or free OSes. Free does not mean bad. In fact, it can be quite the opposite—some of the free OSes have the best support available.
This is easy to understand—most people are not rich and will try to use a cheaper or free OS first if it does the work for them. If it fits their needs, they will keep using it and eventually come to know it well enough to be able to provide support for others in trouble. Why would they do this for free? One reason is the spirit of the first days of the Internet, when there was no commercial Internet and people helped each other because someone else had helped them first. We were there, we were touched by that spirit, and we are keen to keep that spirit alive.
Nevertheless, we are living in a material world, and our bosses pay us to keep the systems running. So if you feel that you cannot provide the support yourself and you do not trust the available free resources, you must pay for an OS backed by a company to which you can turn in case of problems. Insufficient support has often been characterized as an important drawback of open source products, and in the past it may have been the main reason for many companies to choose a commercial product.
Luckily, in recent years many companies have realized how good the open source products are and started to provide official support for these products. So your suggestion of using an open source operating system cannot be dismissed solely on the basis of lacking vendor support; most likely you will be able to find commercial support just like with any other commercial OS vendor!
Also remember that the less money you spend on an OS and software, the more you will be able to spend on faster and stronger hardware. Of course, for some companies money is a non-issue, but there are many companies for which it is a major concern.