Early decision plans for colleges are not without their disadvantages, the biggest being that it forces a decision before the student has the time to know what all of their options and offers may be. This is frequently mitigated by financial aid packages that compensate for the loss of choice, but even so, such plans have the downside of not knowing what other packages might have come to fruition. YC seems to be walk…
In a lot of ways, you could think about this as giving students more optionality as they will have the opportunity to consider YC alongside other extremely compelling opportunities. It's smart for YC to try and get in front of students when they're in a better mindset to evaluate that tradeoff.
I'm sure there are a lot of students who would have be great YC candidates but end up signing a Google offer in the fall and just stay there for years because the pay is so good it doesn't make sense to leave...