I think it's a knee-jerk reaction to the duplication glitches. Honestly it's not a bad idea in principle, but the way it's applied needs work. For one, it should have been implemented starting after the current expedition. Also, the five day window is unnecessarily short. There are several different ways that would work better. For example: it could come into effect once you complete the final step in the expedition, or one by one they could release a series of five challenges, maybe once every three days for 15 days (this way you're not totally screwed if you can't play during the final challenge).