It was this low the 9 months before portal

Was that during Echoes or Revenant? Both were fairly poor quality in all honesty, which has unfortunately become the norm. However, I agree that the Portal isn't the root cause of the problem, much like seeing someone angry at a computer monitor rather than the CPU.

It dropped off about a month after final shape tbh so more like 11 months. FS was actually very disappointing for the game itself as once you did the post campaign bits there was nothing to do. I thought those 2 latter episodes are the equal of edge of fate, in fact there was more replayability in both those than edge. Quality has fallen through the floor now & quality control isn't even a job title at bungie anymore. But hey, they're working on a roadmap of new events featuring old content, that'll fix it, bit it's a month away apparently so might be dead by then 🤞