When a software becomes fragile, it breaks often and most of the time unexpectedly, even if the change you make is a simple one and remotely unrelated. Developers are afraid of making changes to the existing code because they might break the system unintentionally. However, as long as the software isn't abandoned, changes always come along. Making changes to a fragile system would probably make it even more fragile, and the complexity of the system would increase with hacks and corner case covering code.