The argument, such as it is, is that impeachment is the remedy for a Mad King Trump situation, rather than the courts. In fairness, this is not a completely unreasonable reading of the Constitution, but the framers' intent is almost completely irrelevant to the reality of our current political system. As originally written, the federal government was basically designed to be a vaguely-representative oligarchy, with states free to appoint senators and presidential electors however their legislatures saw fit -- the majority of states did not consistently hold a popular Presidential vote until the 1820s, for example. Impeachment by 2/3rds vote is not an unreasonable bar to set when it's assumed that everybody in government is going the part of the class and social structure, and the President acting as a class traitor would put all of Congress into uproar. The founders did not anticipate more direct democracy, the two-party system, or the vulnerability to demagoguery that those things would introduce into the system.
So here we are now, with a nakedly partisan Supreme Court majority holding that the only way to interpret the law is to ignore the world as it is and instead imagine things are still as they were at the end of the 18th century (mostly because that philosophy plays into the hands of the right wing) and pretending that a 2/3rds vote in the Senate is still a reasonable bar, when in fact the present political reality is that you will never peel 12+ sycophantic Senators away from a dangerous demagogue's camp for long enough for an impeachment process to succeed in removing him from power. Of course that's by design, but textualism and originalism paved the road to this ruling.
At this point I'm not even ironically suggesting that Biden should call their bluff and start offing prominent right wingers. The Roberts court is clearly working in the assumption that Democrats won't play dirty with the tools they're laying out for their incipient god-king, and it's looking increasingly like the only way to keep those tools out of their hands is to strike first.
The founders did anticipate direct democracy, the two-party system, and demagoguery. These were much discussed. They weren't able to provide perfectly for these eventualities, which also was well understood at the time.
The constitution clearly doesn't allow a president to be removed from office by a prosecution, but it just as clearly doesn't offer any immunity to a prosecution for presidents and not to mention ex-presidents. There's never been a presidency, including Donny's, where a criminal charge was even contemplated that would have impinged on a president's legitimate duties.
The founders did anticipate direct democracy, the two-party system, and demagoguery. These were much discussed.
...and notably not a part of the constitution they eventually drafted, which was my point. Rather than try to build a democratic system with effective safeguards against demagoguery, they chose to have a system where only "the right sort of person" got a say in the running of government, and assumed that the separations and limitations of power they wrote in to the rest of the document would be sufficient protection against bad actors in that scenario. Now, we have (more or less) representative democracy, but with no additional guardrails to protect against someone like Trump, and SCOTUS is peeling away what we do have day by day.
You can remove court justices via impeachment. They're not impeaching the president, they're impeaching the Supreme Court justices. They're nominated by presidents and confirmed by congress, so it falls on congress to remove them.