The ruling says that 3 of the 4 indictments against Trump can proceed so long as prosecutors make a case that the President was acting outside of his duties.
A president being incompetent or immoral in his line of duty is an issue for voters or congress to decide on. But a White House bogged down in lawsuits or petty criminal charges would cease to function.
If a president assassinates his political rivals I don't think you can say it's, "an issue for voters or congress to decide on." That would be impossible, since anyone the voters wanted to replace the sitting executive or his allies would just be arrested or murdered outright (as an official act, no less!).
Which they can't because official duties haven't been defined.
Trump can now continually appeal his actions were official, delaying charges until after November when, if he wins the election, he can instruct the DoJ to drop the case.
