pretty sure most billionaires inherited a ton of money from their multimillionaire parents to get them started

and your parents are somewhat right. higher grades can get you higher pay, but grades are not the determining factor. colleges like to see other things than high GPA like involvement in community or something.

maybe your parents are from a different country. in china, the 高考 (gāokǎo) will literally determine whether you go to college or not. its like the SAT but 10x harder. so in that case yes your grade would make or break you