The mountain of evidence for it, thus not being an assumption. Do we really want to follow the evangelicals in being deniers of science?
For example, if someone believes human emotions such as anger, greed, and fear can’t be fully eradicated if that’s what science asserts, then one may lose faith in the path to Enlightenment. One might also not believe in the rounds of rebirth because it’s not proven by science, which would be an obstacle on his path.
Science doesn’t assert that. On rebirth you’ve switched from something we can demonstrate to be false to something which is outside of science, and so what science is silent on (rebirth). I don’t disbelieve the claim about Pāli because of no evidence. I disbelieve it because all of the evidence we have points in the other direction. If there were no evidence either side then I would be agnostic.
The point is some things have to be taken by faith.
Do we need to believe these claims about Pāli in order to awaken? Not as far as i can see.