The argument that I have heard against allowing every one sufficient time to finish all their games (by adding an extra hour) is that it then benefits the people who did not get cucked off spawn (for RNG reasons that makes this game a BR).

I don't quite agree with your wording that there is a "much smaller pool left at the end to que" but rather que times could get drastically longer near the end of sessions as the match making system is based off who has the most similar point totals. And so if que times are extended to compensate for allowing all 10 matches to be played, the extra time allotted is essentially voided meaning there is no reason to add it in the first place.