Suppose I went to a party and told the story that my dog could turn on the television. If my wife did not contradict my story you might conclude that she believed the story if several additional conditions were also met: (1) My wife had the same opportunity to observe the dog as I did. If my wife was at work at the times I claimed the dog did his trick, she would not be in a position to contradict me even if she thought the story unlikely. (2) My wife was at the party when I was telling the story. If she did not hear me tell the story, you could not conclude anything from her failure to contradict it. (3) Most obviously, you would have to know in fact that my wife did not contradict me. It would not be enough that you did not know whether she said anything at all.
With regard to the supposed eyewitnesses who could have refuted the miracle stories: (1) we don't know that there were any eyewitnesses who observed every day of Jesus' three year public ministry such that they would be in a position to state definitively that any particular event had never occurred; (2) we don’t know that any such witnesses were around at the time any of the gospels were being written; (3) we don’t in fact know that no eyewitnesses ever came forward to refute the stories, we simply don’t have any record of a contrary version of events.
Of course, this neither proves nor disproves the truth of the stories or the accuracy of their transmission. It simply means that we lack information about how people in Jesus day responded to the miracle stories. Hypothetical eyewitnesses who hypothetically never refuted the stories don’t provide corroboration.
Moreover, I think there may have been some eyewitnesses who denied the miracle stories. Mark 6:5 says of Jesus in his hometown, "He could work no miracle there, apart from curing a few who were sick by laying hands on them, so much did their lack of faith distress him." There must have been people from Nazareth who had known Jesus all his life and had never seen him work a miracle. Could it be that those Nazarenes were expressing their doubts. Could it be that Mark was responding to them by explaining why they had never seen a miracle?
There are also points in Mark where Jesus instructs the beneficiary of a miracle not to tell anyone about it like when he raises Jairus's daughter from the dead in ch. 5:35-43. Some did not obey his instructions like the leper in Mark 1:40-45, but I imagine some might have thought "If this guy doesn't want me to tell anyone, I am not going to tell anyone. After all, I don't want the disease to come back." So isn't it reasonable to think that there were people who knew Jesus during his public ministry but never heard about some of his miracles because the beneficiary had kept quiet? Wouldn't some of them have said publicly that they never heard of the miracles.
The "lack of debunking" argument seems incredibly thin to me, but it is extremely popular among apologists.