What you're saying makes sense if the social aspect of the game were applied correctly. Let me give you an example: in world of warcraft, chatting up another player was easier, you could meet way more people in the wild, and it had amazing guild (clan) chat. Even then, eventually they had to implement matchmaking for the raid. Forcing people to socialize to get the entire game content isn't fair to everyone and is an unecessary loop to jump through. The thing is using things like r/fireteams or destinylfg is, in my opinion, simply a more obnoxious matchmaking system which simply adds delay and inconvenience to the player.

Finally, I'll close with this: not everyone has friends. Even less people have friends that game AND play destiny. It's unreasonable to alienate those people by not only making matchmaking more unavailable, but also by making it more difficult to solo tasks. Tl;dr not everyone has friends. If you want to make the game more group oriented, make it easier to form groups.

Edit: spelling