I agree it is a horrible policy. Why?

1) You're penalizing those who book later. These are typically folks who pay more for their seat in Y than others who book earlier, both as a function of advance purchase requirements for many fares and becuase inventory gets more and more scarce as you get closer to the departure date.

2) You're penalizing those who are your bread and butter - business travellers - who typically book closer to departure date rather than earlier. You piss these people off, and you're screwed for the future, if you do it often enough.

3) Waitlists change dynamically. Even if there were enough status flyers on the list, some do end up changing their travel plans, and thus open up waitlists. You end up telling the full fare Y traveller that the waitlist is closed one day, and the next day (or next minute) #2 Plat on the waitlist drops off because he changes his plans and decides to travel another day.

4) Passenger loads change dynamically. Similar to Point #3, you've got people cancelled confirmed reservations in both BF and Y class, which dramatically impact how big a waitlist should or should not be.

All these things add up to a piss-poor business decision.