I can’t really comment on the current quality of the game in 2023, since I noped out about 4 years ago, but there are any number of explanations.
Lack of new content. I haven’t seen the game discussed much in the gaming press, which means there probably haven’t been any notable content drops for a while. It sounds like the game’s next expansion has been delayed, and the game is being maintained by a skeleton crew while the rest of the studio focuses on Marathon.
Competition. With few exceptions, good games always slowly bleed users as shiny new alternatives attract players. This is compounded if you’re running a live service game and have a long content drought.
My point isn’t “Destiny good”, I don’t really know that. My point is that we can’t really draw conclusions about the quality of the game based solely on missed revenue targets.