Some animals that have been bred to produce milk for human consumption rely on being milked regularly or they will otherwise suffer pain and eventually might die. Completely and abruptly switching to a vegan lifestyle would therefore cause animals to suffer.
once asked a dairy farmer about this and he said that (abrupt) stopping of milking will likely cause inflammations and other complications
A global shift towards a vegan diet is necessary not only to ease the suffering of the millions of animals killed daily, but also to save the world from hunger, fuel poverty and the worst impacts of climate change.
Caring more about animal lives than the level of vegan diet being possibly available to everyone is putting animal lives and human lives on the same level which, in itself, could be considered "humanly" immoral.
Eating vegan CAN be ethical but you can still eat vegan and eat unethically. You can still support bad corporations while buying their vegan options, you can buy vegan good that is not socially conscious (slave labour/gmo crops/come from companies that engage in all kinds of unethical things) etc.
Morality is a finely sharpened tool human's ancestors developed through evolution to help them spot noncontributing members of the group. This is essential for group-work. Sometimes humans can use "shortcuts" of thinking in inapplicable situations getting them to reach the wrong conclusion
Animals and plants have the same value in life as default. The most ethic way to eat is by balancing between plants and animals so that the ecossistem is kept working well and every life form keeps on living without much interference.
A whole foods, plant-based diet is a healthy diet. This means that a vegan person is more healthy. Health renders a state of well-being. Well-being is better than non-well-being. Thus, a vegan diet is more ethical when it pertains to humans.
with each pound of meat given up in the production possibilities curve between meat production and non-meat production there are decreasing marginal returns; in order to continue feeding we would necessarily have to expand farmland and encroach on wild species habitats