I have nothing to add to the medical comments but just wish to offer my condolences. I'm so sorry.
I am so very sorry for the loss of your husband. How terrible for you.
The fact that he passed away so quickly suggests that, as you were told, most of his lungs were compromised by overwhelming infection, not just a small part here or there.
Unfortunately, several of my friends have perished in this way; the infection had simply taken over such large areas of their lungs that neither a ventilator nor antibiotics could save,them. There is a kind of critical tipping point in disease where there really is no help, sad to say, and people can die very quickly when this point is reached.
During any surgery there is always the risk of infection which often results in pneumonia, which is an umbrella term for many types of chest infection. In many cases patients go into theatre with a small trace of Fungus, bacteria or viral infection in the Lungs and when the body is weakened (because the immune system is busy helping with repairs) the infection can grow and take hold of the patient. Microbiologists have to quickly identify which type of infection it is and select the best antibiotics. While waiting, a general antibiotic is used. Due to infection, gases find it difficult to cross into the blood in the tiny vessels in the Lungs, hence the need for a ventilator which forces air into the lungs at a higher pressure than that in the room. They can also modify the oxygen concentration in the mixture. What I don't understand is, they must have seen his oxygen levels dropping when the ventilator was removed, so why didn't they warn you of the consequences and reinsert the breathing tube down his windpipe?
I really am sorry to hear of your loss.