Seriously, great article. I wonder if there is a rule of thumb for the number of ski days before a ski loses its life (torsional rigidity, rebound, etc). I fully understand that this depends on the ski construction, and remounts, etc. But a rule of thumb is just that, a rule of thumb ... cheers!
It all depends on the skier. I have friends who tell me they can't really tell the difference between demo skis. So they can keep their skis until they get tired of the top sheet.
Skiing the exact same ski (new vs old) back to back on the exact same trail with firm snow can be very enlightening. Unfortunately, not many skiers get to experience that.
The degradation of edge grip (torsional rigidity) is very gradual. Takes place over a long period of time. Years for most skiers. Most just adjust and adapt without even realizing it. Until they get on a new pair of ski. Then suddenly they are in love all over again.
The exact number of days very often depends on what/where you ski and how hard you drive your skis.
I am pretty hard on my skis. I usually replace my daily drivers after about 75 days. Hard snow and race skis will be less before they lose that sparkle. Wider powder skis usually get blown edges and other really bad damages by about 50 days.
Mamie runs her daily drivers about 100 days. She came to that conclusion after skiing on two different pairs of the same ski back to back. She didn't believe me initially.
When it comes to other skiers - don't really have a clue.