Two sides to every coin.
Great analogy.
Empty seat at concert.
OK is it empty because
1.) it hasn't been sold?
2.) it has been sold but nobody has turned up?
In case 1. it has failed to attract revenue and that fact that someone else is occupying is it opportunistic but ( and remember it has failed to sell) this hasn't cost the organiser anything. Neither the bloke from the cheapseats nor anyone esle was going to pay to sit there but the seat exists and the concert goes on. So someone makes use of it.
In case 2. The seat is bought and paid for but no-one shows for whatever reason. The seat is has generated revenue for the organiser - he has suffered no loss. The concert goes on. The bloke that decides to occupy it has, presumably also paid for a seat. If the rightful owner shows up he will get his seat and the interloper goes back to the cheapseats.
For any commercial flight the second the door closes ( gate closes or 35 minutes pre departure at T5 ) then an empty seat on an aircraft is worthless. It is a drag on revenue, a cost to the airline. It has failed to sell , yield has not been maximised. The cost of that seat whether it be in row 1 or row 21 is exactly the same. Their respective revenue generating potential is significantly different only until the ticket desk closes. On board, they are indistinguishable - every empty seat costs the same. Arguably, the cost of an empty premium seat is higher as the middle row seats are usually not sold. It would therefore always make sense to upgrade pax if there is a better opportunity to sell an economy seat where there is no prospect of selling premium.
If the seats had anything other than perceived value it would be easy to auction the seats after boarding! Clearly, this is a. not practical and b. would only serve to alienate those willing to pay premium prices for ordinary seats.
So let's just put things in perspective. If I or anyone else gets away with sitting in a 'better' seat it is opportunistic and subject to fair challenge. In the absence of any challenge from CC, with the benefit of pax manifest, seat allocation and loyalty card staus etc. is that down solely to theft/trespass on their part or ineptitude or complacency on the part of CC - or both? Knock for knock.
Finally, I would seriously challenge the assertion that one seat is 'better' than another simply by the placement of a moveable curtain - which more often than not is what differentiates premium from cattle class on short haul flights - I know you will tell me if I'm wrong!
For the avoidance of doubt, whilst the economics of long haul are essentially the same I do regard premium seats as offering more than 'perceived' value on longhaul services.