Hmm... Lots of stuff, lots of attacks and lots of good advice too. But it seems OP still does not get it, still arguing about free/empty seats.

So, OP, imagine that:
1) You run an airline
2) You charge X for normal economy and 1.25X for premium economy (these are hypothetical examples!)
3) You have free seats in premium economy on this flight

Now the question to you is: Would you move someone who paid you X (and costs you Y) to premium economy seat where you have to provide more (hence costing you more than Y - don't forget it is not only more space, but usually other extras too), but don't get any extra money for it?

I bet you would not... So here is your answer to your original question.

BTW, I always ask about possible op-up (that is very different from "complimentary upgrade"), and I have been reasonably successful - got even C to F. However, it was always only and only when the original class was over sold...