Just a claim without any backup.
Destiny is a multiplayer focused game, at heart it even wants to be a looter shooter and hence a long term hobby. If people just play through the story and find nothing to do at max level and leave after just two weeks there is something seriously wrong with the game design. Sure Bungo has the money and they obviously do not care, but it affects the likelyhood of future purchases. Reddit, official forum and press were full of D2 shortcomings like no other game (maybe No Mans Sky). Absolutely everything points to dissatisfaction. You can say you do not believe it but that is just your opinion and nothing else. Looking at the grand picture tells another story. You have absolutely nothing to contradict this thesis.
A general drop in player activity is normal but not at this rate. You may provide sources with other succesful AAA games that focus on multiplayer and worse drops to prove everyone wrong.