>I work for a F500 company

As have I in the past.

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>Will they use this to replace you? I have never seen that happen.

I have. Repeatedly.

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>That conversation is incredibly easy to have with your boss if you have a good relationship with your org.

Your boss has a boss who also most likely has a boss. These conversations don't stop with your boss. Even if your boss doesn't think it's a big deal, chances are good someone else higher up might have an issue with it. And when the time eventually comes to have those conversations about who to keep/let go...those people with reps are some of the first ones to be brought up in those conversations.

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>Technical roles in F500 companies are expensive to backfill.

No, not really. A lot of people want to work in them and budgets are usually larger and they can be more competitive in attracting talent. Smaller companies with smaller budgets/footprints actually tend to have a harder time attracting people due to the above mentioned issues.

Make no mistake, everyone is replaceable. F500 companies are not immune to bad management and the short comings of having people, and the baggage they come with, complicate the process.