That's not how it work in a 2-party system. The 2 parties themselves are changed.

This happens constantly. The Republicans were the party who ended slavery and gave the vote to non-white people, while Sothern Democrats fought the hardest against women's suffrage and civil rights. Hell - 30 years ago we had a Democratic President who banned openly gay people from serving in the military.

Ans as much as we like to blame everything on Reagan, Trump has massively transformed the GOP through his ability to primary his enemies in the party.

The Dems are going through a round of change, and the old guard is scared they'll be primaries next. And it's our duty to make sure that happens to move the party to the left.

Have you seen the abysmal discussions around third parties for US politics?

It’s always the same thing on both sides. Republicans didn’t want to start a new party when the tea party people popped up as it would split the R vote. Democrats don’t want to create a new party as it will split the D vote. The only way to get an additional viable party would be for both D and R to split and add two new parties simultaneously. So, as someone who will never vote R, I sure hope all of those “sane republicans” I keep hearing about start their own new party.