If you cannot be replaced, you are un-promotable. Your entire career becomes tied to a single point of failure. Then, when they replace that piece of software with a new product you have no experience elsewhere.

My career in a nutshell:

- I picked a company that was less than 100 people and in business for at least 2 decades, and profitable these were my core parameters

- I worked like a dog first few years, specifically focusing on picking up everything no one else wanted. And in every shop there will be plenty of this

- I f'ed with the code that says DO NOT TOUCH THIS, NO ONE KNOWS, EVERYTHING WILL BREAK

- I intertwined myself with every part of the product (multi-million lines of code product)

Then:

- At 5th year anniversary I asked for 60% bump and yearly 15% raise moving forward, the owner did not think for more than a minute and agreed

- At 10th year I quit, started my own LLC and offered my services to the company at the same rate they were charging their customers for senior engineer rates (which was roughly 2x what my salary was - no problems)

I made more money that I can spend in 4 lifetimes just following these simple principles. I became virtually irreplaceable and henceforth could demand just about anything I wanted (within reason of course but that 'reason' is very high...)

I always find myself thinking why are so many people trying to get jobs at FAANG/Big Tech where there is a much simpler (and attainable) path that being one of the thousands and practically just a number.

A couple of things:

* Employers do not heavily invest in training. They over invest in hiring/firing. If you want to shine at candidate selection time you need to look like that perfect guy who they can fire with minimal risk and replace you with a 22 year old.

* Employees that over prioritize in retaining their current employment, such becoming that irreplaceable center of attention that keeps all the lights on, are high risk. Nobody likes high risk.

* Software developers tend to prioritize all the wrong things. They tend to prioritize things that make their own lives easier at cost of everything else. You can label that immaturity, autism, sociopathic, or whatever. The result is the same. Employers have real product decisions to make and if you aren't on that same line they will consider you as a potential redundancy for elimination, because you consume more resources as an employee than you deliver as a developer.