But either way, these folks aren’t the ones who ultimately get chosen in the primary because they’re never extreme enough.

I think this is primarily due to the fact that the practice hasn’t caught on or been promoted effectively in deep red states. If you had a movement to get people voting in the only election that matters in their state to the point where moderates started winning, you’d permanently start to shift the policy positions of candidates. Nikki Haley didn’t win the primary but she was only able to hang on as long as she did due to this strategy.