Pat Buchanan won 367 delegates, and 23 percent of the popular vote.
Even further back, Herbert Hoover did not debate Joseph I. France, who won 7 contests and 48 percent of the popular vote. Calvin Coolidge did not debate Hiram Johnson, who won 29 percent of the popular vote.
The people who are claiming that they would vote for Robert Kennedy are likely not registered Democrats. You have to be overwhelmingly delusional to believe that Robert Kennedy would win the Democrat nomination. That would be the equivalent of Charlie Baker winning the Republican nomination.
Furthermore, participating in a debate would likely hurt Robert Kennedy Jr, due to his speaking disorder, spasmodic dysphonia.
The incumbent president never participated in the debate in their re-election bid. Donald Trump did not debate Bill Weld, Barack Obama did not debate John Wolfe, Bill Clinton did not debate Lyndon LaRouche, George H.W. Bush did not debate Pat Buchanan, etc.
The Democrats are the ones who claimed that 1980, 2000, 2004, and 2016 were stolen. Your party is entrenched with hypocrisy. When your opponent wins, then the election is illegitimate. When your candidate wins, then the election is secure.
I assume that would mean that you do not have a legitimate response to make. I am not silencing Robert Kennedy Jr by simply reiterating the points that he has made. I would honestly support Robert Kennedy Jr winning the Democrat nomination, because I realize that he would have zero chance of defeating Donald Trump. Just because he is better than other Democrats does not make him good. One could say that crack is better than heroin and that wouldn't make crack good.