It's an example which demonstrates the concept since in both cases, the overall process/system is unnecessary. Neither dairy farming nor killing animals for meat is necessary. It's not shifting the argument to say that the killing isn't necessary in the first place, that simply is the main point that the fallacy ignores.

With regard to dairy farming, it's not more profitable to raise, house a male calf who won't produce milk in their life, and feed them until adulthood (still only a few years old when they can live until 20-25) and kill them for beef. In most cases male calves get killed for veal, though they can simply be killed immediately and discarded, while some are raised until 1.5-2 years and killed for beef. Most female calves usually become dairy cows and then ultimately beef cows as well at 4-6 years old.

On a mass scale of dairy production, the killing of cattle for veal and beef is absolutely necessary. And yet, these components are part of an overall unnecessary system that is dairy production. Of course it's cruel in a variety of other ways too, but the primary use of the fallacy is assuming that we need to eat/utilise veal and beef due to them being necessary for dairy production, when dairy itself is unnecessary.

I love how everyone jumped on the example I used to defend these cruel practices instead of understanding how they were an example of the fallacy I was describing. And are trying to claim they're not an example of the fallacy when they clearly are. Shows the world we live in...