but I think for so many companies, especially larger ones, you're not paid based on what you're worth
Yep. THey hire based on narrow function of the position, not the person to fill it.
you're actually paid on what it would cost to replace you.
Precisely. This is eloquently put - I'm gonna steal it! :D
Which, honestly, has always worked out really well for me and my weird little niche skills.
Gathering hot women willing to be in front of the camera? :D
Niche is good for many things. I have a few "differentiators" of my own that i can leverage. The thing is, most people want to be the best of the average, so having skills that stand out of the average, is highly valuable, even if they aren't the best.