The graveyards are full of indispensable people.

I'd both agree and disagree with that statement.

There are people who simply cannot be effectively replaced. That does not mean that the powers that be will not replace them. For example, a startup might have a critical engineer who happens to know how everything works or holds everything together. They can be replaced, although this may have a severe impact on the startup, including the failure of the startup.

To your point, even if someone is critical to the success of an organization does not mean that they won't ever be replaced by poor management.

While I agree that "never" is a strong word, we usually over estimate ours and others importance and irreplaceablity.

Sometimes replacement of a critical engineer might even do good to a company by forcing them to break the silo.

That would not necessarily be poor management though. If a single person is irreplaceable, it is a major risk for any organization.

True. But your skill and value is proportional to the cost of replacement, so not all is lost.

Old school advice. Always be replaceable.

you can but that doesnt mean the company can find an equivalent replacement

That doesn't matter to the company so long as the work gets done and the customer keeps paying.