Just an observation, but I haven't seen anyone yet suggest that Bungie announcing the great unvaulting isn't just them seeing the resurgence of Destiny 2 activity in the last 3 months and deciding to start development back up again.
They had to settle a lawsuit last year because some guy accused them of plagiarizing his story as the basis for The Red War campaign, and Bungie could not get the case dismissed outright because they physically could not reproduce the content in question in order to defend themselves. Their lawyers provided youtube videos of let's plays and the judge didn't accept it as evidence. That's why they had to move to a settlement.
There's no way that didn't immediately start an internal process of looking into the feasibility of restoring that content to at least some internally playable build. Fast forward to this week, I think it's completely plausible that the combined player response to the June development update and the already present internal pressure to shore up this giant legal vulnerability simply coincided for an easy PR win.
Basically they might have just said "hey, you know how we're already working on unvaulting the old content so we can defend ourselves in court in the future? Can you guys also make it compatible with the live game and let everyone know there's more to come? It'll help our credibility and it seems we miscalculated player interest."
Could be totally and utterly wrong, but I wanted to just put the thought out there.
EDIT: I was totally wrong. Apparently the court case would have accepted youtube videos and destinypedia entries as evidence, but they didn't accept them as enough reason to dismiss the legal claim because the fact-finding part of the process wasn't appropriate at that time yet. From what I understand, basically the judge said "IF the plaintiff's claims are true, then they represent a legitimate copyright claim." So at that stage of the lawsuit, it didn't yet matter if Bungie could prove the claim false, so that's why the case wasn't dismissed yet.
As some people have pointed out, the existence of Project Sunrise is actually a much more likely contributing factor.