I stopped playing during that Episode because it just sucked, I then came back during Heresy, it was also meh and felt very stale, so I once again dropped the game until the announcement of it shutting down. Not only did the story feel complete with The Final Shape, but the desire to play more was gone. The game had gotten really stale, the seasonal/episodic model was just the same shit every couple months with a new layer of paint. Almost identical systems every season. That fatigue resulted in a collapse after TFS, since I bet I wasn't the old one that felt this way.
The seasonal model was likely a contributor to the decline of the game over time, but as I try to think of alternatives, it's hard to really come up with something that would have been objectively better in every way.
For example, in D1, the initial plan was for new games to be released every other year, major expansions in the off years, and a couple minor expansions to fill out the schedule every few months, so a two-year cycle for each game in the series. It would have kept every game self-contained, but it also would have meant needing to lose your progression and start over every couple years (assuming there was no carryover between titles). Not only that, but the latter updates would barely have any time to play them before the next game came out, so that would be a lot of wasted development effort on stuff that barely anyone would end up playing in the long-term (don't worry, I'll get back to that).
Okay, so what about how D1 actually ended up going: new game release, major expansion the next year, and smaller updates whenever they made enough to add to the game. Kind of the same thing, but you still end up with big content droughts. Like, I know people look back on TTK fondly, but there was practically nothing new to do for an entire year there, with the only update a few months later adding another strike and a revamped Prison of Elders. If there's not enough stuff to entice people to keep playing, then it's going to be hard to keep the series going. And you still run into the issue of what to do when it's time for a new game.
So we come to the seasonal model, keeping the same game going by adding some new stuff every couple of months, then removing it when the year ends. Again, you run into problems. As I mentioned earlier, a lot of development resources ended up going into stuff that wouldn't be in the game forever, especially things that ended up in the later seasons of the year. Some of it eventually stuck around, but most of those activities aren't available anymore. On top of that, a lot of it felt pretty samey. There's only so many ways you can make a three- or six-player activity and make it feel fresh. And drip-feeding the story over a few weeks got on people's nerves, especially when those story bits were artificially extended by forcing you to play a round or two of the new activity before you could get back to the questline.
I think the core issue is that Bungie was investing far too much in this ephemeral content and not providing a greater variety of things to do in-game, which would have sorely helped those content droughts. Look at SRL, for instance, or even something as basic as fishing. It's different from the usual "shoot baddies and dunk balls" activities, it gives you something smaller and simpler to do, and it helps to break up the monotony. Or hell, having a personal space to build up and decorate with the spoils of your adventures. People enjoyed collecting stuff for the H.E.L.M. in various seasons, but it would've been even better if it had actually stuck around and not disappeared when the year was up. (I miss my fishtank and my rock collection.)
I guess the ideal would have been for them to separate story beats from content drops, adding in new questlines or missions when they were ready, same with new activities. Instead of trying to reinvent the Menagerie a dozen times, they could have just focused on something new and memorable when they had a good idea. And having content that filled more niches would have done a lot, I think. (Also not leaving PVP to rot while you moved all those devs over to Marathon would have helped.)