I'd equate genetics to talent and it presents you with your maximum capability and that everyone has a different ceiling.
So genetics has a role.
But the importance of that role may be overstated.
Maybe one person is born with 5k talent to run 12:25 and another with talent to run 12:45.
The lessor gifted person may have more motivation and attain a time of 13 flat while the more talented one may quit before realizing his potential.
Certainly we have seen people with inferior genetics beat someone more gifted before.
Some people take the mindset of the OP and use that as an excuse to not believe that they will ever be very good.
Why was Christophe Lemaitre the first white guy under 10 in the 100?
Because other white guys with the same talent believed they were limited by their genetics while he didn't.
I doubt he is the most talented white sprinter ever.