I really don't think they'd just leave a door open on purpose, just so a team could win.
We know that the producers had a limited number of grand prizes to allocate each season. If you look at Season 3, the wins prior to the Red Sash run were Much-Heralded Helmet of Sir Gawain, Lion-Headed Bracelet of Chandragupta, Secret Map of the Bandit Queen, Ivory Hunting Horn of Roland, and Lost Hornpipe of the Pirate Captain. That means that they had already given away half of their grand prizes for the season, and had already passed the halfway mark for episodes this season. They probably wanted to give the teams a chance to win (or at least do relatively well) since they still had half of their grand prizes left.
EDIT: I want to clarify that, including the Red Sash's taping day, there were 16 runs left for the season. Of those, a good number of them had to be wins to meet the production team's budget (if I recall, there was an interview where somebody had mentioned that there was a limit to the number of grand prizes they could give away, but had to give away at least some number of grand prizes also). In order to "spread the wealth," they most likely wanted to give all of the teams a shot at potentially winning. By the time they got to the last production day, there were still a lot of prizes left, so they made the paths for those runs easy enough so that the teams were more-or-less "forced" to win and take away a grand prize.
Another thing to consider is ratings: Would viewers watch or continue to watch the show if every run was a complete disaster with a ridiculously long path that left the team with no chance of winning? They had already had that occur during the Discarded Seal day, taped two days prior. And then the Royal Torque of Queen Boadicea and Enormous Feather of the Me Linh runs taped the day prior also featured long paths, even though those teams were both pretty successful. Rather than have another day of insurmountable paths, they probably wanted to give the teams some ability to succeed. So even if they didn't leave the door open so that the team could win, it could have been to at least allow them to make for an exciting run that left viewers with some hope that the team would be successful.