'but my gut tells me that all of the people who think "genetics is destiny" are losers.'

Try doing a sport competitively at a high level, where the whole team is eating the same, working out the same, and some plateau at a 315 deadlift and start getting injured all the time and others keep going to 450 without any problems.

And that's ignoring obvious things like deformities, asthma, insulin resistance, etc.