Actually, given the rise of SCEA/ED the rise of yield should follow. As a pure hypothetical excise, consider 100 kids who would be cross admitted to all top ten schools. In previous year only ten of them applied EA/ED and ninety did RD. So each school would have issued 1 early admit and 90 during RD. Here, for simplicity I assume the interests in ten schools are even and all kids admitted in early round were satisfied and not applying in RD (but if they did the results would not have changed drastically). So, in this previous year the yield of the ten schools would be 10/(1+90)=11%. Now, this year the same 100 kids all decided to do SCEA/ED, and again for simplicity they were evenly split among ten schools and no further application in RD. Now the yield this year would be 10/10=100% for these 100 kids.
Given that 50% of admits nowadays are from early round I think any uptick in SCEA/ED among the super strong applicant pool would raise the yield.