> Isn’t it generally accepted that increased diversity in a decision group leads to better decisions?
Perhaps evidence to the contrary, it seems widely believed that the governance of most Nordic countries and other nations like Switzerland result in (much?) better outcomes than the US. There seems to be something of a fascination with those nations in terms of their social support systems and the like. Interestingly, those nations are substantially more homogeneous the United States.
> Splitting the state up would draw territory lines where there are only cultural lines currently.
I don't think it's a given that this is a bad thing. The principle behind states and counties is that the populations can govern themselves in a way that fits their values. An extreme case, an Amish population that has gathered in a particular region, would probably not feel great about being forcibly assimilated into another group's social or governmental structures. Their beliefs are so disparate that it's difficult to imagine they would be able to find common ground.
> What else can we do, is there something less radical that can chip away to close this gap?
Acceptance of other people's values without demonization is key. The US has devolved into a state where the opposing political belief is now seen as a flaw of their character or morality. You see that Even on a site like this where the average commenter is better educated than the average citizen. (The comments about Trump supporters all supporting segregation as example).
To fix this, we need to break the echo chambers that perpetuate these beliefs. Unfortunately, because of the current polarization, it's difficult even for an elected official to agree with the opposition about any issue without reprisal from their own party or the voters.