By the time Taken King came out, me and my friends had already played the game for so long, we just didn't feel like playing more. Especially my friends who actually bought the two little expansions. I just got the base game, so I didn't play as much as them. And when the game started making it so that I couldn't play every daily and every weekly and the top strikes anymore, that's when I started getting weened off the game and I was freeeeeee! See ya suckers.
Oh wait, where was I? Oh yes, The Taken King. Every time that's on sale for $13 or less, I wonder maybe I should go back and play that in single player. My friends played it too much (thousands of hours), they weren't willing to ever go back anymore. But maybe I can enjoy Taken King as a single player game.
When the base game came out, I thought it was excellent as a single player game. Sadly, they kept updating it, and eventually made it so that normal difficulty was too easy, and Hard difficulty was literally impossible because fighting enemies 2 levels above you was suddenly crazy hard instead of just being challenging like it was at launch. That, to me, was the saddest part of the changes of the game. They made it better and better for coop and worse and worse for single player.
So if my friends are unwilling to get Destiny 2 (because they played too much Destiny) then I'm not sure if it will be worth getting for single player. I bet you they'll make normal difficulty too easy again, and hard difficulty too hard.