I recall the C++ "fragile base class" problem already starting to rear its ugly head by the end. Existing base classes couldn't be reasonably extended without breaking compatibility with existing applications, and the BeOS API was routinely designed in a way that promoted subclassing. e.g. to implement a window, you'd subclass BWindow and fill it with your own subclasses of BView. It's difficult to see how they would have dug themselves out of this hole in the long term. Perhaps eventually they would have needed to ship multiple versions of their system libraries corresponding to each ABI breakage.