Having been on the side of thinking I was absolutely irreplaceable on a team, I was more or less proven right when I left and the team failed to deliver anything at all for about 10 months, even having to shut down existing products, and is now limping along.

The truth in retrospect is that it was my fault (and my upper leadership's) that I wasn't replaceable. I created a knowledge silo around myself since I wanted to move fast and figured I could prevent the team from being bogged down in complexity if I just handled it myself and while that worked in regards to delivering out-sized results for the available bandwidth, it also was a risk that materialized as described above. So while I do believe that everyone should be replaceable and it's their responsibility to be, it's not always the case and products can live and die by it.

Saying everyone is replaceable is a first-degree approximation. The next level of detail would talk about, at least: (a) how much knowledge gets lost; (b) how many person hours -and- "wall clock time" it takes to rebuild that knowledge; (c) the amortized cost (or benefit) of losing that person

It depends on how well or poorly run the business is.

I worked at a company where everything hinged solely on one guy working from another country. When he left, loss of institutional knowledge took about three days to show real effects as things also came down crashing.

I worked hard to make _myself_ replaceable for when I left, it was a pretty good exercise, but me having that degree of freedom was symptomatic of the problems of the company.

how can you possibly say everyone is replaceable while next giving an example of a case where it's not only not true but potentially so invisible in its consequences as to make it hard or impossible to satisfactorily replace someone? a "person" can be rebuilt but not replaced. companies and societies aren't actually bose-einstein condensates. everyone is replaceable is just what different types of people say as a coping mechanism. it's a disgusting thing to promulgate too. maybe if someone is equivalent to a cog they can be replaced but humans are not exactly standardized cogs. what you mean to say, and may get in the habit of repeating instead, is that we don't necessarily need a specific person even though we are accustomed to them. it still might take you 10 years to find someone who does 80% of the same things as the other person while the missing 20% is what made that person such a unique hire in the first place. i find it a disgusting phrase that is so typical of the view of a society like ours today that is losing view of the value of life.

The dangerous piece of this is executives who don't have a solid grasp of how things are operating can assume that a product still running after the departures is still successful. Validates their decision making even as things deteriorate in the background.

That said you can replace people and build back that institutional knowledge -- both loss and gain take significant amount of time.