Do you think there is a definitive reason why the game is less populated than before?
I think it's because we don't raid anymore.
Why don't we raid?
I think the reason is simple, we just don't like hanging around with strangers, especially when some of them struggle in the parts of the game that most of us take for granted and do with ease.
We don't like a mismatch of interest and experience in the game. We would raid if all 6 people had the same experience and knowledge in Destiny. But that has become a relic of the past, a unicorn, or simply a dream that we will never see become true, no matter what they do.
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Because its not required in the gameplay loop like to should be. Not required for max power. Not required for the best loot. Etc.