Things are politically stagnant because people believe that politics is about systems. Politics is about power, and politics will always be an expression of the dominant power dynamics. Governmental systems are just how power is explained to outsiders; it’s a mythology that’s told to disguise the real nature of power.
So the question of systems is a red herring, that’s been carefully instilled. This has been true for all history: Many kings don’t really rule, courtiers do. Only kings who can effectively wield power rule, and they’re historically in the minority. This should also be obvious in the US: corporate power is only ever checked in the presence of enormous public action. Not public bitching, public action—general strikes being the most important example.
Or to put it really bluntly, while there’s a lot of pageantry in politics, what politics actually is, is power struggles. But they sure don’t want people to recognize this, which is why there’s so much pageantry and partisanship.
This is also why the government is going so hard against Trump, but letting Pence, Clinton, and Biden slide. It’s not because they cooperated—if you or I had security clearances and just took documents out of a SCIF and kept them at home, we’d be in jail. It’s because Trump clumsily challenged existing power, namely the federal bureaucracy (which he conspiratorially calls the “deep state”), and he wasn’t up to the task.