A big part of the rather documentation and process-heavy development model at FAANGs and other large corporations is to ensure that other people can take over a project if any individual leaves. Much as people will hate this analogy, "cattle, not pets" is a concept managers and HR knew long before the software industry stumbled on it. The people at FAANGs are highly-talented but, even at that level, only a handful are truly irreplaceable (and those people were so valued that they were allowed to work from anywhere they wanted even before the pandemic).

As an individual engineer, I think it's inherently valuable to make my projects easy to hand off (in addition to as maintenance free as possible). The company is less impacted if I leave, sure, and it also means I can take a vacation without constantly getting messaged. Long term, it means that I end up working on more bleeding edge "cool" projects, since I'm not bogged down with years of making myself irreplaceable, and folk want new work to benefit from the same level of redundancy and maintainability.

I believe that I have a perception of being irreplaceable, but it's because of my future value rather than past value.

I've never felt like I was being treated like cattle at work, though, so maybe I've just been lucky enough to work at good companies.

It doesn't really work though, thanks to "move fast and break things". Losing a team member on an important old system is a big loss.

FAANGs have 10s to 100s of thousands of employees. "Vets" are a lot less irreplaceable than you might think.

Technical debt is technical debt.