Well sure. But yield management is something that colleges (and for that matter the entire travel industry) have understood for a long time.
Let’s take a concrete example: Carnegie-Mellon, an elite college with some departments among the best in the nation.
Using numbers from collegdata.com, 24351 students applied, of which about 4140 students were admitted, and 1805 enrolled, giving a yield rate of 44%. But that includes ED, which is nearly 100% guaranteed. If we back out the 345 ED students from both admitted and enrolled, we end up with an RD yield of 1460 enrolled / 3795 admitted = 38%.
CMU admitted 109 students off its waitlist, or 6% of its class (it’s unclear how many of them enrolled). So even though they had to admit more than 2.5x the number of RD students as they needed to fill the class, they were able to get close to the number they needed to fill the beds. Also, realize that the ideal number of waitlist admits is small but not zero. The waitlist is useful to fill unexpected gaps in the admitted class. Perhaps not as many humanities students enrolled as expected, or they need to fill an orchestra spot. The waitlist bails them out.