No license can guarantee future software availability. Although a copyright holder can traditionally change the terms of a copyright at anytime, the presumption in the BSD community is that such an attempt simply causes the source to fork.

The GPL explicitly disallows revoking the license. It has occurred, however, that a company (Mattel) purchased a GPL copyright (cphack), revoked the entire copyright, went to court, and prevailed [2]. That is, they legally revoked the entire distribution and all derivative works based on the copyright. Whether this could happen with a larger and more dispersed distribution is an open question; there is also some confusion regarding whether the software was really under the GPL.

In another example, Red Hat purchased Cygnus, an engineering company that had taken over development of the FSF compiler tools. Cygnus was able to do so because they had developed a business model in which they sold support for GNU software. This enabled them to employ some 50 engineers and drive the direction of the programs by contributing the preponderance of modifications. As Donald Rosenberg states "projects using licenses like the GPL...live under constant threat of having someone take over the project by producing a better version of the code and doing it faster than the original owners." [3]