If you read the second paragraph the authors explain what they mean by "becoming".

>People in advanced economies stand to inherit around $6trn this year—about 10% of GDP, up from around 5% on average in a selection of rich countries during the middle of the 20th century. As a share of output, annual inheritance flows have doubled in France since the 1960s, and nearly trebled in Germany since the 1970s. Whether a young person can afford to buy a house and live in relative comfort is determined by inherited wealth nearly as much as it is by their own success at work. This shift has alarming economic and social consequences, because it imperils not just the meritocratic ideal, but capitalism itself.