Supply and demand. He's right, but the question is when.
I had a graph of this for a college project, I can't find the graph atm but in the late 1980s there was 1.6 million suckler cow's in Ireland and 1.2 million dairy cow's. In 2015 I think it was, the numbers had basically reversed. The number of dairy cow's has increased since then.
The beef industry suffers from oversupply. If every lad goes into dairy, and every existing dairy farmer expands, you'll vastly increase the supply of milk. The demand for milk is actually going down thanks to veganism.
If the number of dairy cow's just kept expanding, eventually there would be an oversupply of milk, and as Mf240 says, the arse will go out of it like it pretty much has done with beef.