Ah, yes, I see. So you're not saying that a plain plurality election avoids the extremes. But yes, you're right, we don't have a plain plurality elections, because they are obviously broken. Instead, we have primaries to narrow the choice to effectively two candidates, and then a general election that is essentially just deciding between those two.

It's a more nuanced question whether this avoids extremes or not. It comes down to human behavior, so there's no answer from logic or mathematics alone. In practice, I think what we see is that when one party settles on a broadly appealing candidate, the other party chooses a moderate candidate to remain competitive (e.g., Obama/McCain or Obama/Romney), but when one party chooses an unpopular candidate, the other chooses a more divisive or extreme candidate to exploit the weakness (see Clinton/Trump). So it can have a moderating influence, but not always.