Actually, my parents have been not allowed in a lounge when they were entitled to use it. They weren't that bothered, I wasn't that bothered on their behalf.

Stuff happens when you travel. Sometimes it's big and important, immensely inconveniencing... that's when you ask for compensation (luggage loss, huge delays, bumps, involuntary re-routes etc etc etc). Not for really minor stuff like this.

If you had answered oh they had to buy x do y as a result of not getting in the lounge then I would have understood you better. But you talk only in terms of opportunity cost, not in terms of actual losses. So it appears your parents didn't really lose out...

With all the travelling you must do to have earned all of those top tiers, you must have experienced many, many occasions when things didn't go as they should. With the amount of travelling you must do, I'm surprised you haven't already realised that worrying about little things like this is going to kill you - since minor problems occur many times when travelling. It's surprising to see such naive attitudes coming from one so apparently well travelled!