For meat breeds, the veals stay under the mother. If it's for veal meat, they can be bred very late without being weaned at all nor having a diversifying diet (and have iron deficiencies for it but they're slaughtered soon enough), to maximise the mass of veal meat. Basically, they're raised by the mother. Maybe formula if there's a problem with the mothers production but not otherwise. And these breeds are not milked for human consumption anyways so .. ?

For dairy breeds, considering a good holstein makes 50L of milk per day and a veal drinks 2-5L per meal (2x a day) depending on age, you've got a lot of milk left. And since most medications make milk improper for human use, this is the milk that's given to a calf, past the first milking (colostrum, which is too fat to be suited for consumption anyways), bc there will most probably be at least one cow under medication at any point of time. Otherwise they take classic milk.

For high value breeds, like for specific cheeses or so it's worth to buy formula. But for your basic milk pissing holstein, probably not. I mean, formula is not free.

Weaning is not 8 months anyways. I guess you could, just like some kids are breast fed until 4 yo... But a few months is enough. At 8 months they would not feed 100% on milk anyways, that would be an always shrinking part of the diet. So even in that scenario, the mother would still produce from 10-20L of milk and the calf would drink maybe 2L over a day.

The narrative of the calf deprived of its mothers milk to feed humans just shows a laughable lack of knowledge of the milking industry, yet that doesn't stop people from talking out of their asses. Breeds are now so proficient they far exceed the needs of their calf.