The Division made the same mistake as Warcraft and many other MMO-like games before it. The raid endgame was comprised of one nigh-impossible Incursion (group dungeons) that awarded the very best loot in the game. Then the next update came out and all the new equipment was better than everything you spent the first few months working for. With a snap of the fingers the developers trivialized the entirety of your previous playtime.
It's no wonder that most people quit and never felt like going back. It left a sour taste in your mouth.
No doubt some people left the game cause of the raises in gear score and whatnot but to consider new gear being a mistake... Hardly. Its what the game revolves around. New gear so you can do the same area over again slightly quicker and slightly easier. Its the core concept of loot games and without it you would be done with them much sooner than you realize.
Releasing game modes as expansions is more of an issues for sure... DIII didn't release Rifts as an expansion. It was just a new way to loot gear but the Division took the Rifts concept (The Underground) and released it as a paid content. It certainly sucked, the game mode as a expansion, but the real issue with it was the unholy grind that they put into it for some reason. It was unneeded and they've gone back and removed some of it but it was certainly too little too late. Also at some point matchmaking for the Underground broke and people just moved on from it after Massive failed to address it. (The match making might still be broken for all I know.)
Survival, another game mode. This time it was season play boiled down to an hour and a half with roguelike/survival game elements added. I rarely played it but it was an interesting idea and I'll give them points for it.
Finally the last "expansion" was again another play mode. This time proper/true PvP in the Last Stand which is by far the most annoying of the expansion basically because it was something that could have truly built the playerbase back up if it had been part of a free patch. People think map packs are a horrible way to split the community try just locking all of the MP away behind a paywall. Anyhow, its actually a really solid mode. I'm shocked that I like it as much as I do but that paywall means its got some serious issues. Likely 20 plus minute waits while you hope the room will fill so the game will kick off and you can do nothing while steering at this screen.
Honestly, if they had released a PvE only DZ with tougher enemies, created a few named bosses to randomly populate an AI squad of rogue agents, and made Last Stand free (and released it earlier) they would have retained so many more of their players its scary. Hell, it likely could have been another R6S and came back from the dead.
Oh, and if they would stopped trying to balance PvE and PvP using the same

ing stats. If a cheap F2P game like Marvel Heroes can have skills doing two different functions based upon rather or not your doing PvE or PvP I cannot fathom how a AAA publisher like Ubisoft cannot manage to do the same thing.