I think you have misconceptions around how much people want to pay for documentation . Usually people come across MDN docs while googling for some keyword . At times MDN is the first search result, but if the users hit a paywall, they will gladly go to the next search result - usually w3schools . It is not a definitive resource but is good enough for most general use cases.
As an example , Wikipedia is used much more than MDN docs as everybody in the world can find something useful on Wikipedia but MDN docs appeal only to web developers . Yet every year; Wikipedia has a hard time gathering donations to run their site with a skeletal crew and they have to resort to huge banner messages exhorting users to donate.