EA was over 12,000 but keep in mind that some of those would obviously have been ED had the option been offered in those years. That’s what happened this year. 13,000 total divided into something like 8000 EA and 5000 ED. (Basing those guesstimates off the admit rates thrown around these threads for the past several months now . . ). If overall early numbers IMPROVED over prior years (which they seem to) and a good representation of those are ED, that means in prior years there would have been eager ED’s applying as well. UChicago knows those numbers - they would be the kids who committed very early on in order to get priority in the housing queue.
This year there is still an incentive to apply EA if your other top choice is an ED school (Columbia, NU, Penn, Carleton . . . there are others) or you also want to apply EA to Notre Dame. That way, your options to apply early decision to UChicago are still open in case of deferral. EDII is not just for those kids who have applied SCEA somewhere else.
The big difference this year - as we all know, is the 4,000+ drop in apps due Jan. 1. Obviously, some of those who applied RD in previous years would eagerly have chosen EDII had they been given the chance. This year they WERE given the chance so whatever number of new EDII’s there were this year would be no lower than any previous year. That’s not the source of the drop. The source - again, as everyone knows, is in regular old (or new LOL) RD applications. Why would they have dropped?
@FStratford attributes it to complexity and he/she might be on to something. Would argue that it’s not “irrational behavior”, however. Students and their families interested in the elite set of colleges tend to be very savvy about admission plans - they DO search the web, read CC, etc. and incorporate what they’ve learned into their decisions. Highly doubt anyone was truly “confused” about the switcheroo this past year (as Admissions was reported to have said) but they likely were surprised by the news and perceived it as a negative shock. Perhaps expected likelihood of getting in RD was now a lot lower, given that there are three admission decisions before March (two of them being ED), so why bother? (Parents could have played a huge role in that, btw, simply by refusing to sign an ED agreement). Perhaps UChicago appears as harsh, calculating and not nurturing in the right sort of way (Ellison letter) and to add two ED options on top of that seemed offensive so No Thank You. The question is whether those who DIDN’T apply would have had higher stats or been higher quality applicants who would have gotten in. We don’t know the admitted stats yet (we have Nondorf’s word, but he’s already on record as exaggerating a bit so . . . ). However, even if the stats are the same or a tad lower, it’s not clear whether Admissions was willing to trade that off for something else - a more diverse class, a LOT more firm commitments, etc. @JHS haSo . . . hard to know. What we DO know is that they accepted a ton of ED’s (I and II). Could have been an act of desperation - or it could have been very calculating. Evidence points to it’s being the latter, given that they were clearly willing to go aggressive on the number of EDI accepteds, well before the deadline for RD (which they extended in part, btw - anyone remember that? 
).
This quandry looks as if a whole bunch of extra kids whose abilities, interests and goals aren’t a good fit with UChicago simply didn’t apply this year. They were obviously turned off by some aspect of the college and/or the application process. If UChicago admits a whole crap-ton of ED’s again, then the signal is - so what? If they pull back on ED - even at the risk of a drop in yield - then the signal is that aggressive ED doesn’t give them the class they are looking for. This can change year-to-year, too, of course depending on the quality of applicants. Highly doubt it will spiral down to Zero RD’s simply because there are high quality HYPS/SCEA applicants out there who will apply to UChicago in the regular round. Will it PRIMARILY be that type? Perhaps. And maybe, given the number of ED’s who are currently the Class of 2021, that’s all UChicago really wants to be in the RD round.
@JHS at #26: it’s obviously a cynical exercise - we disagree on the degree to which other top universities are also cynical. But anecdotally, there are a couple others this year who could have planned their expansion in a classier fashion and were very public about their “oopsie”. Almost as if this kind of news isn’t exactly bad for admissions.