| I like how the naysayers are depicted as sober, rational minded individuals while those who see things progressing more rapidly are shown as crazy lunatics. They are both making predictions about the future. Why is one claim more valid than the other?
It's because the naysayers are the ones more actively working in the field and closest to the experimental and theoretical results and are trying to actually accomplish these kinds of tasks.
Obviously in 1895 heavier than air flying machines were possible because birds existed. And in 1895 there was a significant science & engineering community actually trying to do it which believed it was possible soon. Internal combustion engines of sufficient power/weight were rapidly improving, fluid mechanics was reasonably understood, and it just took the Wrights to re-do some of the experiments correctly and have an insight & technology about controls & stability.
So in 1895, Lord Kelvin was the Kurzweil of his day.