"But I want to play World of Warcraft offline too!"
No.
It's about the experience, and the technology required. They're using server technology (and possibly some sort of streaming service). You can't access those things if you're offline. And part of their game is that you encounter other players while you're playing. It's not meant as a solo-experience.
Destiny is in the first place, if we have to believe Bungie, a FPS and not a MMO, where you have to play with/against other people.
Also people online are jerks, look at the teabagging, team killing, team shooting, the screaming etc. in Halo alone. That is not a really good experience in my opinion. So sometimes I would and I can imagine others would prefere to play alone/offline.
Destiny, if you listen to bungie, is not an MMO and I didn't say it was one. They do, however, use MMO-like server structuring.
Go play Guild wars 2. See how they did it. I have a feeling it'll be almost exactly like that.
But you can "play alone" but you'll be playing alone, with other people. And seeing as the main game is coop, not competetive, team killing more than likely won't be allowed.