OP isn't wrong. All the important characters do have bloodline traits.
To be fair, IRL, you still need good genetics even if you train/study so this isn't unrealistic.
Magic like devil fruits or high tech gene science is the only way to overcome natural genetic limits.
Genetics can only get you so far
You can come from a family of out of shaped inbred trailer park Alabama red necks but through practice and training you get better and better until you're the best.
The same can be said vice versa you can be born to a family of Olympic gold medalist but if you're lazy, don't put in the work and do nothing but eat all day those athletic genes won't Matter in the long run you'll just be out of shape and weak with no skill
Genetics may make it easier or harder but effort is what's important without effort Genetics is nothing
Just like without the material to build a house having the blue prints doesn't matter