Even just killing all a billion would be quite expensive by comparison. You can’t exactly do it for 0.01$/ cow and fixing this would likely cost significantly less than 10 million.
Not everywhere and especially not anytime soon. India has more than 3,000 institutions called Gaushalas maintained by charitable trusts that care for old and infirm cows. It’s a whole religious thing.
Also, enforcing rules isn’t free. Trying to enforce a cow ban would get really expensive.
