You will probably replace the camera body you buy now long before any of the current camera manufacturers retire their systems (or go out of business or whatever). With the rate of improvement and innovation in camera electronics right now, one generally doesn't consider a camera body to be a long-term purchase. A bigger concern would be the accessories you buy within that system. Lenses (and to some extent, system-specific flashes) can easily be the bulk of your investment. In fact, "can" is too weak — if you're serious about photography at all, they will be and should be. And lenses should last through many generations of camera bodies. So, if your ability to use them with a new camera were to be limited, that'd be unfortunate.

However, I wouldn't worry too much. In the event that something changes, a) you will likely be able to get adapters, which is rarely ideal but should still let your equipment work, and b) there will be enough people interested in keeping going with the system they have that your gear should keep a decent resale value for a little while, which would either let you switch to something else — or you could be on the other side, buying up people's stuff as they bail out. In other words, a system won't vanish in a day.