IAN-UK

You miss the point completely. Nobody is talking about costs to the airline. We refer to the expectations of passengers.

If a frequent flyer comes to expect frequent and regular upgrades from cheap fares to the business cabin, then he/she is unlikely to buy business-class tickets. A situation unloved by the airline.

LH would, I think, far prefer the upgrade to be a rare event whetting the appetite of travelers to the delights of the forward cabin.

However my own history of upgrades is so very low in percentage terms, I feel the OP may have been over-egging his history - or I am flying the wrong routes at the wrong time!