Change happens over time, the supply and demand chain would slowly require less animals to be bred for human consumption. (Right now 150 billion land animals every year are breed just to meet the demand.)
Checkout the webpage below to really put things into perspective : no graphic footage!
(http://thevegancalculator.com/animal-slaughter/)Meaning no more unnaturally breeding animals (Artificial insemination). They can have their own reproductive cycle back and have natural intercourse again.
Some people enjoy keeping animals as companions in their homes or on santuaries, so I doubt they would all go extinct as soon as you think.
Overtime perhaps we will let them roam like wild buffalo in national parks.
Just because humans would no longer be breeding the animals for food doesn't mean they are useless.
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