No, I wasn’t born yesterday. Quite the contrary–I’m an active old Penn alum who’s been observing Penn Admissions for literally DECADES (and more decades than I’d like to count 
). I also have a kid who recently went through the Penn Admissions process, and I’ve read and heard Dean Furda’s comments on year-to-year variations in application numbers on several occasions.
Your theory makes absolutely no sense and, indeed, sounds like the speculation of someone who has no knowledge of or experience with Penn Admissions over the past couple of years, let alone the past several decades. Why in the world would Penn suddenly decide for the first time in 2015, NEVER having done so before, to defer the application deadline just to goose the number of applications? The number of applicants rises and falls CONSTANTLY from year to year. Yes, the general trend has been for the number of applications to rise significantly over the last 20 years, but there have been several years when the number of applications has fallen a bit from the year before (as occasionally happens at virtually all of the Ivies), and Penn has NEVER deferred the application deadline merely to increase the number of applications. And neither have any of the other Ivies. In fact, the very few times that I recall Penn deferring the deadline were either when an unusually severe weather event made it difficult or impossible for many applicants to submit their applications by the deadline, or when the Common App was having glitches and “issues” causing Penn and other peer schools to defer the deadline because of the difficulty experienced by many applicants in getting their applications submitted by the original deadline.
Not to mention that your explanation utterly fails to account for the other schools–Duke, Dartmouth, Chicago, Vanderbilt, and possibly others–that also have extended their application deadlines this year, and also have never done so before simply to goose the number of applications.
And, of course, there’s also just plain old common sense. Do you really think that extending an application deadline at the last minute by FIVE DAYS, in the middle of winter break, is a strategy that a highly selective, Ivy League university–not to mention several of its highly selective peers–would suddenly pursue for the first time in its long history just to increase the number of applications? Do you think that the Admissions Office would conclude that applicants who otherwise decided NOT to apply to Penn are going to wake up on December 29th and think: “wow, now that I suddenly have an extra five days to submit my application (including all the necessary score reporting, recommendations, etc.), I WILL apply to Penn!” And how many additional applications do you think they’d conclude would result from that sudden five-day extension? Would it be enough to overcome the potential loss of credibility they’d engender in the process? Would it be enough to significantly decrease the acceptance rate? Remember that we’re talking about tens of thousands of applications here, so there would have to be an increase of THOUSANDS of applications to reduce the acceptance rate to any measurable degree. And are you aware that acceptance rate constitutes a mere 1.5% of the total score used by US News in its rankings? A five-day extension announced on December 29th ain’t gonna help much there.
Sorry, but absent significant additional evidence supporting it, your explanation runs counter to the longstanding admissions history of Penn and the other peer schools that extended their deadlines, and counter to plain old common sense. At least to those of us who WEREN’T born yesterday. 