Not necesserally;
In a situation where neutrality is very difficult, or where all partie have very much to win AND the means of controlling what is printed, later studies might be more reliable.
I mean, in your example of the WW II books, both the NAZIs and the Allied Forces had to keep the moral high, while not releasing information usefull for the enemy. When the fights are over, and when there is nothing to win in distorting the truth, a more accurate version is possible (and this indeed might be decades after the facts).
Now, I'm not saying thet the quran is more reliable than the bible (or the other way round). I'm just pointing out that your argument doesn't apply to all situations.