Again, you are assuming that Congress and SCOTUS will stay consistent in their behavior when there's a new President. The exact same Senate and House that exists today will impeach the next Democratic President in seconds should he/she repeat 1% of what Trump has done since taking office. And every executive action of theirs will be blocked by the Supreme Court in a 6-3 vote.

I think you should expand your line of thought. Think like MAGA in this situation. If you are a Democratic politician with no scruples and a drive to implement your agenda, what could you do? Assume for the sake of argument that you were swept into office on a tide of anti-Trump backlash and you have a majority in Congress.

You could start by passing legislation and excluding it from judicial review under Article III. After all, as you say, the SCOTUS would otherwise vote along their own ideological lines against everything you want to do. Sure, SCOTUS and others will undoubtedly howl that Marbury gives the court the right to judicial review, but you would not be the first president to ask the court "with what army?"

We are at a crossroads. Will the Democratic Party see itself as responsible for conserving the republic and push the government back towards something boring and sane? Can such a party actually get elected today? I have this suspicion that a lot of people think so, especially MAGA -- they [mostly] cannot conceive that the opposition can turn the tables and use identical tactics on them, so they feel like the current situation is a temporary but crucial win only for them, which will move the Overton window to the right. But what if there is really a sea of anger boiling below the surface right now just waiting to be tapped by a Democratic demagogue?

Could get exciting.

I don't think so, I think I'm accounting for the recent shift in realpolitik. Why would future Democrats impeach their own POTUS for using the DOJ and FBI to arrest their political enemies?

The Democratic party as you knew it is dead; it died in 2024, just as the Republican party as your knew it died in 2020. The Republican party has been reformed into the MAGA party, which bears no resemblance to the neocon Republicans of the 2000s. Just the same, the Democratic party will reform but they will not resemble the party of Clinton/Obama/Biden/Pelosi/Schumer. They are done as a political force.

Moreover, why wouldn't a future POTUS start off by arresting the current conservative SCOTUS judges? Decide on the arrest, make up a pretext, if US attorneys don't comply just fire them until you find one that does, like what they're doing to Comey right now. Make some vacancies and then appoint his own court. Or, just ignore them entirely, there are no consequences for not following their orders.