Everyone seems to be using the logic that 'we have all at some point been recipients of upgrades so why deny others this opportunity'.

Conversely, as someone who always ends up on flights that are almost full but never over sold, and has never been upgraded, what should my reaction be?

I flew MEL-DXB earlier this year as the only passenger in F, while both J and Y had only a couple of spare seats. Being the only passenger vs having several passengers in the cabin does make a difference in terms of attentiveness of the crew. I didn't pay for the whole cabin, but if I'm getting something I didn't pay for (nicer cabin ambience), why would I want it taken away?