In regards to the Destiny 2 power fantasy... At launch it basically took the same time to kill enemies at max level with decent gear as it did when you were leveling up. Add the crazy long cooldown on ultimates and it never really felt like you were getting stronger. Add in the fact that you hit a point where they only way to progress was to wait a week for your powerful engrams to roll over and D2 had no sense of progression or power fantasy.
Re: Diablo 3 - At launch Diablo 3 was a very shallow game. The difficulty curve was stupid and the RMAH short circuited the gameplay loop. Once loot 2.0 came around and Reaper of Souls launched, D3 was actually an ok game. Had they kept up with content updates and thought outside the box a bit it could have been a great game. Unfortunately at that point the brand was tarnished and they decided against future expansions.
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This is slightly false. You need 4 players to complete a stronghold. At least at launch. There is built in matchmaking however, so you won't be posting in random reddit threads to find a party. *cough*Destiny*cough*
Everything else can be done solo or in groups of up to 4.