I don't think it was a wrong move to start a partnership with Activision. They poured, or at least had planned to pour like five hundred million dollars into Destiny.
It was obvious that Bungie didn't know how to develop a loot-based online RPG and if I remember correctly, it was the Diablo guys who helped them to fix the game and release The Taken King. Having the help of Blizzard and other Activision studios was certainly a merit of working with them.
Although, I believe it was Activision's push to release a sequel called Destiny 2 that ruined everything.
In the new blog post released by Bungie after revealing their future plans for Destiny, it's mentioned that releasing Destiny 2 was a mistake and that actually set them back for more than a year on their original plans. They didn't want to split the players by releasing another game, and that's the plan right now: to keep updating Destiny 2 forever and bring Destiny 1 content in it too. I think they plan to merge both games in the future and go back to their original idea for the game: one game, called Destiny, updated and kept alive for years. Kinda like World of Warcraft or something.
Activision is repeating the same mistake with Overwatch 2. Their push to release standalone sequels is definitely not helping. I remember Jeff Kaplan saying in the Overwatch 2 announce video that he had to fight hard to keep the pvp part the same in both games, while making sure that the sequel is appealing enough to be sold as a new product.
Bungie did the right thing. They don't have the marketing push of Activision, and maybe that's why we're not hearing about Destiny as much as before, but I'm pretty sure Destiny is successful enough to fund other enormous projects at Bungie.