> If there was a pressure valve where another party can simply take over
That's exactly what happened though -- the MAGA party took over. Conservatives "cut bait" with traditional Republicans, condemned them (see how they talk about Liz and Dick Cheney or even GWB, Mitt Romney, and John McCain, their own presidential nominees), and started a new party within the rotting corpse of the old GOP. There's still some "Republican" branding around but if you pay attention they're not waving "Republican" flags or wearing "Republican" hats anymore.
It’s not a good safety valve though - sure, in a 2-party system, one of the parties can be taken over from within. But it’s uncommon and hard to achieve, and risks alienating voters while the civil war is going on.
On the other hand, with IRV or preference voting, second parties can form without spoiling the vote for their ideologically most aligned alternatives. This allows for a much more seamless shift.
Really in the US there should be at least 4 parties formed from the corpses of the big two, if not more.
Unfortunately taking over a dominant party was the easiest way to have a "different" party that could actually win. Both parties have built a mountain of obstacles to prevent a third party from ever getting close to challenging them.
I find it best to view parties like any other faction or gang. They don't want challengers to their current power. Primaries are supposed to be the democratic way to steer a party but we've seen how that goes. They aren't going to change unless it is from within. So,remove all obstacles to being on the ballot and let the existing parties whine about it when they start to lose.
That's exactly what happened though -- the MAGA party took over. Conservatives "cut bait" with traditional Republicans, condemned them (see how they talk about Liz and Dick Cheney or even GWB, Mitt Romney, and John McCain, their own presidential nominees), and started a new party within the rotting corpse of the old GOP. There's still some "Republican" branding around but if you pay attention they're not waving "Republican" flags or wearing "Republican" hats anymore.