Indeed, you can buy Linux support from Red Hat and other companies. But this rings hollow - they do not provide the level of support that Sun or Microsoft does - try getting them or anyone else to fix an obscure bug in the GNU linker, especially if you're not using their distro (even if you want to pay). If you think you're going to convince the likes of prickly GNU toolchain developers to prioritise the development or release of a patch your company depends on, guess again.
And those support contracts cost money, just like Sun/MS support contracts. And as soon as you start relying on software that isn't in their distro, you rapidly run into uncharted territory. The Linux meme of 'you can get support', is, I think, promoted mostly by people who haven't worked on critical IT systems, or have done so without understanding their responsibility.
That's not to say Sun's engineering is better than OSS (after all, we got their compiler to segfault as well as gcc, and my BSD/Apache shared hosting has had better uptime than my landline and TV for the last few years, and I'm sure many people want to set fire to the source code for NFS). Linux and other OSS provide the bulk of features that most people use. And you're probably better off with an unsupported apache server than some of the hideous commercial webservers that used to be around.
The bottom line is Sun charges a lot of money to mitigate somewhat nebulous risks that are hard to understand and communicate, and their OS has no killer features. They have also mastered the art of alienating potential enthusiasts. All of which put them on shaky ground. It is a shame, because platform diversity is one of the best defences of the internet as a whole.
(If you have access to dtrace, learn it. Understanding what it does and being able to use it will make you a better and probably richer developer).