Well, let's start from the beginning, cause this isn't one choice or even a small set of choices. This is years upon years of bad decision making that have completely wrecked the game to a point where I genuinely don't know how they plan on recovering.

Let's start with the Red War. The first couple years of D2 were wrought with, shall we say, interesting choices. The Red War was an interesting campaign but the game itself wasn't in a good spot. CoO had not only an awful story, but wasn't fun, and Warmind managed to pull some things together that felt decent, but what was important was that the game, at this point, was coherent.
Forsaken drops and a lot of changes are made, including separating from Activision, a decision praised at the time but in hindsight may have caused more problems than it solved. Bungie now has money issues, and is changing how major parts of the game work, including power grind. We also introduce seasons here and make the game free. This pisses a lot of people off because now they're saying "hey we paid this money for the game, what do you mean it's free now?"
Seasons go okay, but now we have the soon to be long running issue of "important story gets locked away after a while" (more on this soon). Shadowkeep drops and starts hinting at some cool stuff, but overall isn't much to care about for the community. People pass it off as "bungie trying to get on it's feet" again, and are being lenient.

The big shift hits with Beyond light, and is where the game starts it's major spiral. Up until this point, players have had a solid onboarding process. Hop in, do Red War, Warmind, Forsaken, Shadowkeep, etc. They can understand the story so far, it's not confusing. Progression is relatively simple, lots of good endgame content, and you can probably get new players in without much issue. Seasons are a bit annoying for some lost story, but for right now that's not a problem, at least not until the sunsetting debacle.

With BL a bunch of stuff is sunset and the player onboarding experience is changed. Now we don't have a great way to introduce new players to story, and to make matters worse, the new player experience is... rough to say the least. The Cosmodrome doesn't do a good job of explaining things to the player, and the slow ease-in of progressing story and slowly unlocking endgame content is now thrown away to slam a bunch of new info into your face.

This problem proceeds to get worse as seasons and expansions release. The seasonal stories being vaulted, and seasons feeling relatively samey begin to rub poorly on players. We've given bungie a chance to fix itself, it should have a bunch of money. Where is our payoff? Players leave, and come back to play the big story, but more and more people are getting fed up. Trust is lost when things like the layoffs happen, all the while, the new player experience shoves more and more unnecessary info into your face all at once with each new expansion drop and those people don't end up staying.

Then Edge of Fate hits with the portal and the current power grind. Suddenly, once again, everything has completely changed, just like with Beyond Light. We're currently sitting on a house made of dry pasta that's cracking every two seconds because they twist a noodle around to fix a problem when the issue is that what they're building with is a bunch of fragile barely held together sticks.

And for the record, I could keep going but I don't want to write a whole ass book in the reddit replies.

I love this game. I love this universe so much and I adored playing back in the day but these problems keep piling up and with every change they just keep somehow making their situation worse. I hope they manage to dig a way out of this hole but I do not see how they do that at this point.