Literally any half competent debater could have torn Trump apart up there.
The failure wasn’t the moderators but the opposition candidate to Trump letting him run hog wild.
If Trump claims he’s going to end the war in Ukraine before even taking office, you point out how absurd that claim is and that Trump makes impossible claims without any substance or knowledge of diplomacy. That the images of him photoshopped as Rambo must have gone to his head if he thinks Putin will be so scared of him to give up.
If he says hostages will be released as soon as he’s nominated, you point out it sounds like maybe there’s been a backroom tit-for-tat deal for a hostage release with a hostile foreign nation, and ask if maybe the intelligence agencies should look into that and what he might have been willing to trade for it.
The moderators have to try to keep the appearance of neutrality, but the candidates do not. And the only reason Trump was so successful in spouting BS and getting away with it was because his opposition had the strength of a wet paper towel.