All software has a lifespan. The three primary factors that control this lifespan are the popularity of the software; it’s usefulness, and the lifespan of the controlling company. Whilst the first two factors are common to all software, the third is one of the primary differentiating factors between proprietary and Open Source software.
A measure of the success of Open Source software is to gauge the likelihood of the software to survive, if the originating company and/or individuals suddenly ceased all support for it. Is the community surrounding the software strong enough to keep it afloat, or is the community also dependent on the software authors contributions?
Comments by Mark Shuttleworth, founder of Ubuntu Linux, whereby he states that Canonical (his company and origin of Ubuntu Linux) is now just a small part of the Ubuntu community, are a good example of how software can out-grow it’s origins and effectively sustain itself. Such a scenario is often quite difficult to achieve however, and software must certainly be both popular and useful in order to have any hope of surviving. But it is worth remembering that irregardless of how popular and/or useful proprietary software is, it’s lifespan will always be limited to that of it’s originating company. This aspect alone demonstrates that if you must be dependent on any software, a preference for Open Source software is a much safer option in the long term.