His ability to focus on this pattern, to be interested in it per se, is one of qualities of his work that makes him stand out as a writer. But if the rank economical maneuverings of the English bluebloods seem uncomfortably harsh today, in AT's times that sort of idea was just coming into fruition. For most of the history of humanity, marriage has been primarily a social and economic institution. Melmotte is the lever whereby Trollope will pry open the box in which the rules of these games were kept. It's hardly any different today.
People still marry within their own classes.
And kids still listen. It seldom can be otherwise. In the original outline of the novel, Melmotte was a minor character.