Even if Li Jia knew that Du Shiniang's box was filled with a million-dollar treasure jade, and his parents were strict, they would be able to have food and clothing in the future. Li Jia can't be loyal to his teacher's mother all his life, because Li Jia is a rich boy and a dude, and his feelings for Du Shiniang are true, but once he conflicts with his lifelong interests and family status, he will be shaken. He can't be beyond his time, and he can't be divorced from his living environment. So Li Jia can't be loyal to Du Shiniang all his life.
So, is Du Shiniang's tragedy accidental or inevitable? From the analysis of the development process of the plot, there are many contingencies in Du Shiniang's tragedy, but these contingencies are the inevitability bred by the social environment at that time. In the traditional moral concept, the dependent status of women and the humble social status of prostitutes, as well as the relationship between people with interests, are incompatible with the pure feelings pursued by Du Shiniang, resulting in the inevitability of Du Shiniang's tragedy.
Maybe you will say that Du Shiniang has an unknown treasure chest. She can redeem herself, or find another confidant, or go boating in the rivers and lakes, but she chooses to sink into the bottom of the river. Why? We can understand this problem in this way: what Du Shiniang pursues is not a short-term joyous love, nor a formal marriage, but a real feeling of cherishing and respecting each other. During her long career as a prostitute, she was bullied. When she finally got better after hard work, she was cheated, bought and sold, and abandoned. In this social environment where personality, dignity, conscience and emotion are traded as commodities, her ideal of pursuing true love was shattered, so she chose to fight with death.