Hardware may fail. Bugs may remain undetected for years, and suddenly become activated. Besides, software may age poorly, especially if changes somewhere else interfere with old working software. The operating environment may contain risks that undermine sound design, impementation, and operations. Lowest-bidder efforts are intrinsically risky.
Commercial software is commonly far behind the state of the art.
But there are significant benefits to be gained from using certain software engineering techniques. There are inherent risks in using computer systems in critical environments. These must be continually reexamined. Critical and open discussion of critical systems and critical environments is essential. We as technologists must better understand the risks and their implications.