Interesting topic many have gone over but I'm still interested as to the specific reasons why people have left. I've been playing since beta, I'm a good player (top 5% or so in most stats according to tracker)
Myself, I've stopped playing apart from Trials on weekends sometimes because honestly for 6 weeks on 3 characters I got nothing from hard raids. It's like beating your head against a big rng-brick wall that even when something chips off, it turns out it's a 311 chest.
My fix would've been that nothing lower than previous could be dropped (idk if this is impossible to implement) but if you've previously had a 314, it should only be able to drop 314 and above for you. This seems fair. Again, its rng but then you know the grind will actually get you there EVENTUALLY. Sadly skill only now works in Trials which is why its all I come back for (and iron banner clash was fun) but PvE is not even worth the time and effort, especially with all the other amazing games out recently (personally I'm a fallout guy)
Challenge mode changes absolutely nothing. I'll still get unusable drops. At least in previous expansions if you got an armour piece it was always great the first time.
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For a game that revolves around loot there is a major lack of it. Mix in prolonged grind to increase play time with no content I grew very bored. Removing strikes, making previous end game obsolete and taking away actually fun weapons sure didn't help either.