Actually think that @FStratford’s point is the better one here. Chicago’s EA preference rate was pretty soft; maybe 1.2-1.5X overall rate–similar to MIT numbers, rather than the approximately 2x-4x SCEA advantage at HYPS. That suggests that the EA pool may not have been as strong, something that seems reasonable to me. In the prior scheme, although EA at Chicago was non-binding, an early application prevented one from applying to HYPS, and required withdrawal of a Chicago EA application (and potential offer of admission) if the student was admitted ED anywhere else.
As a general matter, I make the reasonable assumption that the best candidates are within the SCEA pool, although I wonder that the overall ED round pool (limited to non-HYP Ivies and, perhaps, Williams, Amherst, and Pomona) might be stronger. Regardless, Chicago didn’t have access to those candidates until the regular round. Chicago has not yet consistently won cross-admits from HYPS, and previously had no idea if ED candidates might rank Chicago higher than Columbia, Penn, etc.
ED does, indeed, put a thumb on the scale, which is a market incentive for someone to become an early adopter. From the university’s perspective, it seems like a win if it can attract: (1) some strong candidates who would have applied SCEA but for the absence of Chicago ED (e.g. for a strong candidate–prefer a 80% chance of Chicago ED vs. a 40% chance of HYPS SCEA); (2) Some great ED candidates who would have previously made calculation #1 in favor of Columbia, etc.