You are heavily exaggerating how complicated Destiny's game world is. Its scale is rather small by open world/open exploration standard. A LOT of games do tons of things you describe in the background while doing 60fps.
Also in patrols the map is heavily instanced, it does not calculate everything on the map at once, only the part you are active in, and other fireteam member's activities are only synced to you with results, it is decided by Bungie's "Activity Host" system where a barebones script machine host an activity for a player and then constantly host migrate between players (source: stated by Bungie in a post Destiny 2 reveal interview, I think it is glixel). No sane network architect is going to calculate everything on the map at once even if the player is not interacting with it, especially with P2P based system that does a lot of workload locally on the client. Again, an issue that could easily be fixed by offloading those calculations to dedicated servers which Bungie does not even bother to care.
TL;DR: your argument that "Destiny does a lot of shit in the background so they are very CPU limited for 60fps" is shaky at best, and could be circulated by using dedicated servers.