Un-enrolling the /bottom/ 50% would probably improve a department, sure. But how do you get that bottom 50% without enrolling all of those students in the first

Un-enrolling the /bottom/ 50% would probably improve a department, sure. But how do you get that bottom 50% without enrolling all of those students in the first place? Your second point assumes that the relative percentage of "not crap" remains constant as enrollment increases, which is highly dubious. And there's a lot of smart people being wasted at Google and Microsoft, too.