Once a particular task is repeatable, documentable, and doesn't involve much decision making, it can and should be moved lower down on the food chain. Because if it isn't, management will one day figure out that they aren't getting good value for the senior person's salary.

The exception might be a very complex mission critical system, such as a high volume trading application. But it's important to note that a company wouldn't likely be betting it's entire business on one individual (which would make him/her a single point of failure, not that it has NEVER happened), so we are still back to 'replaceable' status.

Back in the early days of my career, an instructor tried to impress us with the need to write concise code and include plenty of comments. He then said that in his company, many people thought that not doing so would give them job security, but it actually gave HIM job security fixing things long after they were gone.