I've been saying this on like every page of the thread in gaming side but I really think this decision comes down to technical issues with the campaign being used as a way to introduce features and mechanics to the player as they progress. Because of that, attempting to play with someone who has different progression than you would lead to an asynchronous co-op experience.
Yeah, I think you're spot on about the asynchronous nature of the campaign influencing this decision. A good example of this is Bungie's comments about people receiving an exotic weapon early in the campaign. Deej has said on record that many players quit D1 before they even received an exotic anything so Bungie wanted to let players experience them earlier in D2. That kind of strict loot drip only happens if the campaign is a tightly constructed leveling experience, which is the opposite experience to D1. Regardless, people are severely overreacting. At this point, Bungie is damned if they do and damned if they don't.
"Whoa! Two randoms joined me while I was fighting with Zavala. That kind of seamless co-op gameplay never happened in D1. If the missions are all like this, count me in."
^ I saw TONS of people post this sentiment about Homecoming. Yes, I'll concede that those players might've been referring to the overall feel of the level, but a good majority of them also cited how cool that seamless co-op gameplay with Zavala felt. The current line of reasoning (Bungie is unbelievably stupid if they don't include co-op campaign on the first play-through) seems ridiculous. People are losing their minds for no reason. Wait and see, people.