If the quirk is fallible, random chance implies that in the past there would have been a situation where the collective belief in a better future would have changed it, especially in a life fighting AfO in his prime. This explanation would only have made sense if Nighteye were a kid that had discovered his quirk like last month, or an adult with a normal life.
As for the second part, imagine this situation: I see someone walking into a pit. I tell them that if they keep walking, they will fall into the pit. They answer back that they will not fall because they believe they won´t, and keep walking in the exact same direction. Is there any reason why that person wouldn´t fall into the pit? If this someone (say midoriya) tried to change direction the future changing might make sense, but what happened in bnha is that he didn´t fall into the pit, because it turns out you can walk on air if you believe you won´t fall (looney toons logic).>>191013251
"Nighteye saw All Might die in his Foresight. And even though Nighteye tried his best to fight that outcome, he didn´t truly believe he could do it because he regards his Foresight as infallible, 100% truth and cannot be altered"
That´s from >>191011856
, and in no way implies that the future changed even once before he met deku, and thus had evidence that fate can be changed by belief, except he forgot about it.
And as for the manga itself, pic related. When your only argument is denying reality, you´ve already lost.