Upgrading an operating system sounds simple until you try to do it in a highly regulated environment. In a bank or a hospital, a major OS migration isn't a quick weekend update. It is a multi-year gauntlet of regression testing and compliance audits where one misstep can break critical application stacks.
Ever wonder what happens to a piece of software when the people who wrote it just stop showing up? In the industry, we call this the bus factor. It is a morbid name for a very simple metric. It measures how many key developers would have to be hit by a bus before a project becomes unmaintained. If that number is one or two, you are looking at a single point of failure.