> If there's a sizeable proportion of people like that, they'd be able to get their candidate in the primary.

This was the focus of my post, though I guess it wasn't clear...

What you're suggesting doesn't work across party lines. The Democrat primaries don't care whether Republicans would have voted for Sanders over Trump, and the Republican primaries have a similar problem. Primary systems acts to amplify the passionate voices within each party at the cost of bipartisan preferences.