Adsense was great when it came out (as someone who used it). Passive income, build something and make money in the hours in between when you're not working on it.

I wonder how much more money their (or any) channel would make and how long it would take to taper off?

I suppose the main reason would be algo changes on recommendations, then age after a number of years, and possibly other channels potentially 'recycling' the content. Perhaps the popularity of lego and YT itself also.