You’re right that if it’s one’s minhag there’s nothing wrong. However supporting the boy is not a minhag, at best it’s minhag hamakom. For generations, the boy had to have a way to bring food to the table and did not expect support.
My point about polygamy was just to point out that not everything that was always done, continues to be done. Besides for which it expired. But yes, it is besides your point about supporting the boy being akin to dowries.