Keep in mind, if we don’t eat meat, we will need much more land for vegetables to be planted, so there would probably be less land for cows to get their grass from.
Also, feeding all these animals in sanctuaries would be very expensive if we were not profiting from selling them as meat after.
The question would also apply to all the other animals that have not been mentioned yet, such as sheep, with a whapping 7.5 sheep per person in New Zeland alone.
Also, releasing 1.5 billion cows to go where they like would probably cause some traffic accidents as well as other unwanted problems.
Finally, some people are already making plans for turning insects in to food, as there is already a food shortage in the world. If we keep our meat and vegetables, we can just about feed the 7 billion people we have with the fertile land we have at hand. Soon we are expected to hit 9 billion people on the planet, if we are all vegans, there will be even less spare fertile land.
Personally, I think the realistic view is all the animals would be killed, to prevent them walking on to highways and what not, or if they are released, they would damage the environment.