Yes I have heard of yield. What you seem to forget is business and first class tickets are usually paid for by non price sensitive customers. Usually corporations (that's why its called business) and rather well off people.
Ever seen a $99 fare in business? No and that's why your argument is a little off.
Underpants, I can't say for sure but I would say that a full business and first class cabin would yield more than all off economy?
The point being though is I would say any airline with premium cabins would much prefer to fly schedules that appeal to the business travellers as full biz and first cabin with only half of economy fIlled would be a much better yield than having a full economy cabin with nobody up the front.