I don't get why there is actual complaining about that outcome, because that seemed what the actual intention was. SO made it clear, that they do not want to be a Q&A forum, but a collection of novel and interesting questions. Naturally, a lot of newly asked question were already asked in a similar fashion, which for SO philosophy meant that they could be merged. In addition, they gradually increased the "quality" level new questions need to be have, to be kept.

The declining number of questions is hence not a side-effect or a mistake, but just means that they reach their end-goal. It's actual a measure of success. (Now you can disagree, whether that philosophy was reasonable or sustainable in the first place, but given there choice, this is the logical conclusion and end goal.)