Your question is outlandishly unrealistic and will never happen, as what is actually happening is that we as a society are seeing an increase in both adoption of plant-dominant diets, as well as ever increasing demand for animal products. The world will never go vegan over night, it is, and will continue to be a gradual process.

By contrast, when Biden was in office there were regulations in place to protect us from the spread of infectious diseases - particularly the h5n1 bird flu virus which at present can only spread from animal to human but not human to human. So far it has a 50% death rate for those infected, and it was estimated that if it did become a pandemic, it would result in 150 million deaths worldwide.

Part of those protections involved killing millions of chickens at a time whenever they would get infected with h5n1. So even if your unrealistic scenario somehow magically came to pass, it would still be less horrific than existing reality where they are slaughtered by the billions every year, because at least it would only be a one-time situation.

Three problems with your point about the worst being shut down. First, no they aren't. The vast majority of animal products still come from factory farms. The worst is not some niche, it's the default, because those are the only kinds of farming systems that are efficient enough to meet the extreme demands of consumers.

Second, so-called "happy farms" are a myth. For example, you could look at the investigative journalism of Joey Carbstrong for one example of abuses revealed at these kinds of farms. Every time even the most idyllic farms are actually investigated, abuse is revealed.

Third, even if the mythic pastoral happy farm were real, it still ignores the fundamental awfulness of treating sentient beings as if they are products. Of exploiting them for labor as if they don't have their own lives they want to live. Of forcibly extracting their secretions, of keeping them in captivity, forcibly breeding them, and ultimately of cutting their lives far short of their natural lifespans. It is wrong to kill animals, and especially when it is utterly unnecessary.

Btw, you never answered my questions.