The term “trial by jury” in both the Sixth Amendment and Article III has always been understood to mean what it meant at common law at the time of adoption of the Constitution. As such, there have always been petty crimes or offenses for which a trial by jury was not included in these two provisions of the Constitution. This is why Stevens begins his dissent in Lewis by noting that he does “not quarrel with the established view that only defendants whose alleged misconduct is deemed serious by the legislature are entitled to be judged by a jury. But in my opinion, the legislature's determination of the severity of the charges against a defendant is properly measured by the maximum sentence authorized for the prosecution as a whole. The text of the Sixth Amendment supports this interpretation by referring expressly to ‘criminal prosecutions.’”
One of the primary injustices the right to a jury trial was intended to prevent was a prosecutor or judge silencing a political opponent by throwing him in jail for an extended period. This is not a problem in the US today. In fact, especially when it comes to petty crimes, I think that today judges might be less likely to unfairly convict than juries would. After all, at trial prosecutors are able to persuade the jury to convict in the vast majority of cases; judges, who have seen and heard it all before, and have seen and heard much worse on any given day than most jurors ever have, might be less impressed by a prosecutor’s song and dance. I believe most judges try their best to be fair, have no desire to fill jails and prisons with people who don’t have to be there, especially for petty offenses, and are quite willing to deny even their DA golf buddy a conviction when s/he does have a case.