when either party does achieve a trifecta they very swiftly moderate what has been portrayed as their "full agenda."

In US national politics, I suspect that the least secure incumbents in Congress are also those that are closest to the "center" between the two parties, and that when they're replaced, they're often replaced by similar "centrists", so the median ideological position of the parties doesn't change much.

I also imagine that under parliamentary systems in which people vote for parties rather than individuals, this isn't the case.