I'm not convinced that the number of questions asked is the correct metric. In the end the point is not to have a constant flow of questions, rather constant flow of answers found.
There is a point in proficiency in language/library/whatever after which it is faster to find the answer in the code/documentation/test example than to wait until another person on even higher level will come and answer your question.
Maybe we simply filled out what was needed to be asked in the beginner-bug found-intermediate space and, apart from questions stemming from new versions etc, SO does not need more questions?
Expectation for everything to constantly grow is unrealistic