To your founding fathers thought of this argument I would add that they weren't sure that they had gotten it right and I'm not sure that they did either. I think they did a pretty good job, but I'd rather not find out.
One of the checks is the parties themselves and he has completely overwhelmed one of those (and the more organized one nonetheless). The power of the president is a mix of hard and soft power. So if local police do what he says (or mayors tell them to) then he can control local police. The Republicans have shown that legislators (at least about half of Republicans) are principally concerned about getting elected and so he need only win an election to show that he can flex some might vs them.
The courts are both the hardest and easiest nut to crack. They have been treated as the final word for a long time now. But they weren't always. Also, they tend to take 5-10 years to resolve where to have lunch and even once they've done that a president can always say, "yeah, too bad, I'm not doing it." No president has done that in a long time, but the court relies (almost) entirely on the executive to carry out its orders.
I'm not saying it is likely, but I wouldn't completely rule it out.