Actually your wrong, I am living proof that you can do well without good grades.
I'm not saying you have to have good grades to be a success, but the chances of being a success are far more likely with better grades. It also depends highly on yourself as an individual and your environment. Someone without good grades has a better chance of more progression in life, if they have more confidence within themselves. Also, finding a job at the right company which you can progress in, also is another factor of people being able to become more successful.
It doesn't always just boil down to the grades you got but many other factors also help.