Sorry for the confusion. Winning on Crucible is possible without backdoor, absolutely. I don't like playing 1 v 1 or 2 v 2 against a team that I know will give me a fight on Crucible though, because the game will take 10-20 minutes longer than it would on Prison because that one 4 tower wall is a bottleneck it's hard to get past even when you have catas.
I have found myself on the winning team a few times but stuck at that spot, because even though my team is higher level, our grunts are higher level, and we have flag control the other side has upgraded tower health and damage enough to hold us off.
I like the map alot, it's elegantly asymmetrical, I just think that line of four towers needs a slight adjustment so that they can't all bring power to bear at once.