I would bet its rarely a case. We like to think that we are irreplacable, but after our quit company/project is still moving on.

I agree that life goes on, but it's not rare at all. It happens all the time. People leave and things are f-d up for a while.

The thing is, it's not a permanent condition. People adapt, new hires are eventually found to fill the gap, or others who were thought of as not up to the task take the opportunity left by someone's absence to step up in their career.

One person leaving won't sink a company that is large enough. That doesn't mean it won't hurt, however. And how the company handles things after the departure is very critical.

Perhaps more important (and dangerous) is the morale hit that the others will feel as their peers start leaving. There very much can be a "rats-leaving-the-sinking-ship" effect that can be precipitated by a key person leaving.

Oh, yes. One person does not sink a ship, but I've seen cases where whole teams leaving destroyed companies.

And while I was under no illusion of my importance and did not expect any breakdown of the team or anything like that, I did think I would get at least ONE question after I left from my previous teammates about some obscure thing that I thought only I knew.

Nada. No one asked me anything - they had no issues for which they had to reach out to me. Team did a re-org and things just kept moving. It was a humbling realization to say the least.

More likely no one cared.

It also depends how regimented the team is, places like Amazon have large numbers of coders on the same project, watching over each other. But they also compensate their employees well.

Over the decades this has proven itself to be true so many times. I've been part of big enterprise organisations as well as smaller startups that had some "vital" proces which relied on that one person. When they left however, it didn't really change anything in the big picture, sometimes it let to the entire guts of a company getting ripped out and replaced by something different, but in the big picture it just became another thing to solve.

I think it's a good thing to learn, both as a manager but also as an employee. Because at the big picture, what "irreplaceable" really means is "liability", and that's not a great thing to be at a company.

Good managers will work with you on how to reduce your importance, and great employees will know how to do so themselves. In the world of work, a good work relationship is one where the organisation you work for give you opportunity to grow, and sometimes that means, outgrow the organisation itself. But it goes both ways, and you should try to respect that, even if you're the rockstar employee because it'll make you even more valuable.

they are fine with throwing their money around until a problem is solved. and would much rather hire someone with 1.5x your pay than to give a 10% raise. its just a political / power thing...

internal politics and preserving imbalances in distribution of power is much more valuable than adding a little inefficiency to worker output

In the rare occasion that a single person is valuable enough that if they leave, the company would shut down, the person is well compensated.