> There is survivor bias among groups - if a bunch of people all have different opinions and aren't willing to modify them, they're unlikely to remain a group for long.

I suspect this is why over time all members of a political party become near carbon-copies of each other. Even if the party started "big tent" over a single issue, natural forces will cause it to coalesce into "same thought on all issues".

One solution for the individual is to forcefully prevent yourself from associating with the group, but this often requires drastic action and gets you shunned by all groups. Compare the vitriol against someone who "votes wrong" vs "votes third party" vs "refuses to vote at all".