They don't have lifetime appointments - they have appointments during "good behavior." Who's going to determine when they've started behaving badly?
I mean, the history of basically every country that has fallen into single-party rule is the story of good intentions, including the belief that people will listen to the constitution, not counting for much in the face of real-world political power. I'm not saying it will happen to the US; I'm saying it's naive to believe it can't happen.
If you mean that you're treating the Democrats and Republicans as a single party, sure, there's a decent argument for it, but also, it's a "party" that permits significant internal dissent and debate. The Communist Party of the Soviet Union remained a single party until the end, but the country started on its path towards becoming freer and less authoritarian when they became open to internal dissent.
Frankly, independent of actual political goals, the most concerning thing about the state of US politics is the extent to which the Republican Party does not tolerate internal dissent. There was a lot of it in early 2016, and it all evaporated as people fell in line behind a cult of personality (a term introduced at the start of the above process in the CPSU, incidentally). Regardless of whether you're left or right or center or anywhere else, if you're at a place where disagreement with the leader is a reason to question your loyalty and good faith, you're at a place where there's far less pressure for the leader to serve the country than for the country to serve the leader. (This is why it's not particularly helpful that the leader isn't super competent or even that the leader is not on team More-Stupid-Wars - a position that, by itself, I agree with. I also enjoy it when trains run on time.)
