I'm doing my 2nd play through on legendary, and I noticed something while watching the cutscene at the end of the 3rd mission. Our ghost suddenly intercepts a message from the Witness to Calus. Knowing that ghost gets possessed by the Witness at the very end if the campaign, it dawned on me that the Witnesses dialog from that early cutscene seems like he knew Calus would fail and we'd be there to stop him. This makes sense since the Witness originally had no plans to use Calus and seems to tread Calus very different from his other disciples, as he seems to treat Calus as a disposable tool. Given that the Witness has connected with our ghost and possessed him before, it seems likely this was his plan all along to access the Veil. So, literally, had we stayed on Earth and fought the good fight, we could have won. This also begs the question if this was why Osiris was so obsessed with finding the veil since he drank Nezarec and is likely under the Witnesses' influence. I mean as soon as we land on Neomuna, Osiris main goal is to get us to the veil at all cost and all throughout the campaign he regularly tells the Cloudstriders to send us on these mission rather than having these mighty warriors whose whole existence is to defend Neomuna.