Destiny 2 is somewhat Solo viable, but it is still in his core a Multiplayer Game. If i want to play a straight Solo experience, there are much more enjoyable Shooter games out there that are focussed on Single Player. Also, some of the Activities in D2 are much more fun and easier to deal with a Fireteam than Solo (especially some of the (Heroic) Public Events are bloody cancerous, if you doing them alone) and for later Endgame Content like Nightfalls or Raids you need anyway a group.