> There are other things that can bring a site down, like security issues, or bugs triggered by unusual states, too much traffic, etc.

In my experience as a data engineer, unusual states are one of the leading causes of issues, at least after something is built for the first time. You can spend half a year running into weird corner cases like "this thing we assumed had to always be a number apparently can arbitrarily get filled in with a string, now everything is broken."

Also, conditions changing causing code changes is the norm, not the exception, definitely in the beginning but also often later. Most services aren't written and done - they evolve as user needs evolve and the world evolves.