If you are rich, you own the means to make more money. Your son will then take over from you, and the class structure will remain intact. Likewise, if you are poor, you can only afford to work in a factory, you cannot pay for a good education for your child, they'll end up working in a factory to survive. The class structure remains the same, each generation of rich people will exploit every generation of poor people. That was also why Marx thought the workers must unite and take control of the means of production, so that they would not be exploited.