I turned on monetization a few years ago and I think I've made maybe $20 so far. For a while, I actually considered investing time in making more videos with the idea that the present value of a large number of videos might be pretty considerable over the long term (say 30 years or so).
A few years ago one of my most popular videos (tens of thousands of views) was flagged as being 'not family friendly'. The video was a screen capture of me scrolling through a notepad.exe list of documentary titles with no audio. It was just a list of nature/space documentaries that were already on YouTube. Nothing offensive or controversial.
I guess this is the final nail in the coffin for my YouTube career.