So I think that depends on who you are.
Colleges like Middlebury get a lot of frivolous applications, meaning applications where realistically the applicant was not within the range of qualifications it normally takes to get admitted, and without anything else unique enough to warrant special consideration. For fairly obvious reasons, they don’t get as many of those frivolous applications in ED–like even if some of those people apply frivolously ED somewhere, they can only apply frivolously to one place ED, but can apply to many places frivolously RD. So logically colleges like Middlebury will get a multiple of whatever frivolous applications they get ED when they get to RD.
But those people don’t really have a 9% chance of admission (or whatever the RD acceptance rate is). They basically have a 0% chance.
Meanwhile, there are other much stronger RD applicants who Middlebury actually wants. But these are people likely to have other competitive choices, because they are strong applicants. So these sorts of applicants Middlebury needs to admit a lot of, because they are not going to yield at a high rate. And so these people don’t have a 9% chance of admission either, they have something much, much higher.
OK, so when you look at these acceptance rates, the right way to think of them is that if Middlebury accepts 39% out of ED and 9% out of RD, that just means that 39% of the ED pool are the sorts of applicants Middlebury wants, and 9% of the RD pool is. Which is consistent with that observation about frivolous applications.
But that has nothing in particular to do with your individual chances. The right question for you as an individual is whether you are the sort of applicant Middlebury wants. If you are, then they will admit you. If not, they will not. And the idea when you apply affects how they see the answer to that question is not necessarily well-supported by this data, because there are alternative explanations for those statistics.
Indeed.
From the colleges’ perspective, the question is going to be how many of the enrollees they want in their target class can they get out of ED. That is a function of how many such people actually apply ED. If they get enough great applicants in ED, they will be able to get more of their class out of ED. If they get fewer such great applicants in ED, then they will be able to fill less of their class out of ED.
OK, so there is a sort of loose tier of colleges that have ED and apparently get enough great applicants in ED to be able to use that to enroll a majority of their class through ED admits. Middlebury is in this tier, and so are a number of other colleges.
But in the greater scheme, this is still just a narrow slice of the overall US college ecosystem.