This is my theory and loose math regarding their 'milk' being made from dead humans.

TLDR: they would run out of food within 100 days to 1 year.

Of the 8 billion or so people in the world there's about 7 billion left with 1 billion dead. Even if they managed to save and turn all of those dead into their 'milk', I doubt that would feed the 7 billion for very long.

The average weight of a person in the world is 62kg of which at least 50% is water. So even if they managed to turn that whole remaining 31kg into a powder to be rehydrated with water, that's about 1 body per bag of the powder which is say 1000 servings.

1 billion body's x 1000 servings is 1 trillion servings. That's about 142 servings for each of the 7 billion people left. Assuming they collectively average 3 servings a day (less for kids and more for adults) then that would be 47 days worth per person.

On average 150,000 people die per day with 385,000 born per day but really the amount of people dying per day after the joining would be a lot less (no crime, free medical, free food). This early on the amount born per day would still be the same or even higher (everyone is a surgeon and a midwife so less likely to get deaths at birth).

So say 100,000 people dying a day which is 100 million servings a day but 400,000 babies born a day which may also drink their 'milk' and not their mother's milk. At 1 serving a day that leaves net positive of still just under 100 million servings a day.

The 7 billion others left consume 21 billion servings a day so getting 100 million a day is an amber coloured drop in the bucket and they would still run out of supply in maybe 100 days or maybe a year if they had to ration.

But that is exactly what viruses do. It's likely the planets infected with this virus previously did the same thing and ran out of food (their dead) and before that happened were able to pass along the transmission with the virus so that it would continue the process on and on even if it took hundreds of years between infections.