There's a mechanism by which Congress can remove the president if he gets out of control.

That's ultimately the result of the threats FDR made to pack the court if they didn't do what he wanted.

There's really no way around the possibility that whatever you've written down in your constitution will be ignored in the heat of the moment, or become degraded over time.

But it's not that the duopoly is disappearing. It's just that the previous two parties are being eclipsed by two different parities. That's occurred previously in both the UK and US.

The argument isn't that it helps the US create good law. It's that it keeps the US from creating too many bad laws.