Of course, unices are secure in their usefulness and longevity as well, including Linux. After all, unices are the corpus of the Internet. If unix disappeared tomorrow, the Internet would go with it, and Microsoft doesn't currently have anything that could fill the void. As the major open source example of unix, Linux too is secure, and is rapidly expanding to fill more and more roles, and to have a greater and greater presence within those roles it already fills.
Being both free and high-quality makes Linux very desirable.
In a very general sense, open source development essentially is the next killer app or, more to the point, the current killer app. It may be a couple years or so before a majority of the industry realizes it, but awareness of this fact on some level is growing like crazy. The more traditional "killer app" examples cropping up all over the place are just the end results of that parent killer app, the open source development model.
Look at Firefox, OpenOffice.org, MySQL, CVS and Subversion, the GIMP, BitTornado, Apache, VNC, and Perl for a few examples of "killer apps" that are open source development standards. Some have been around for years and become daily parts of the IT life, and some are reasonably fresh and new. Expect more of the same, and notice that Linux is among them.
Keep Up with TechRepublic