Robustness is orthogonal to versioning. They were funding opengl dev on osx, and decided to stop. You can insert your own reasoning but I believe the more reasonable assumption here is that they were not happy with the direction the spec was going. Apple is strongly biased towards vertical integration. Owning the spec + OS + driver + hardware is the best way of achieving a high level of robustness (Whether they actually do achieve that remains to be seen).