I agree that there is no 'absolute' salary mark that would dictate what class of fare you bought (within reason - you need to have the dollars). It's more about what you want to spend your money on. I have had people at work quite incredulous at me travelling J or F, saying what a waste of money (though some are quite understanding). I then ask them about the money they spent on their holiday houses, their classic car collections, their obsessions with going to the (Gold Class) cinemas 5 times a week, and remind them that I don't wish to spend my hard-earned on these things, that is their choice, and flying (when I can) in a premium cabin, as opposed to the cheapest fare of the day that they will take, is my choice of how to spend my money.
I was many years strictly a Y flyer, until ~2001, when I redeemed a J journey to NZ. Since then, through a combination of points and discount fares, and allowing that there is not in the context of AFF a huge number of trips involved, only once have I flown flights longer than ~4 hours in a class lower than J (MEL-SYD-HKG-MEL on QF in 2007, and I put in unsuccessfully for a points upgrade for that as well), and also many shorter flights in J. Oh, there was one JQ *class return trip as well, I guess that counts as PE.
Only once in that time have I paid 'full price' for a premium cabin, and that was VA MEL-ADL-MEL, as a specific condition to be comped Platinum.
I won't rule out paying full fare premium in the future, and I won't rule out long-haul Y travel (though I wish I could

), but really, there are many creative ways, as we all know, of travelling in premium cabins without paying full price. And for me, most of that knowledge has been gleaned here on AFF. Sorry if I have drifted a little OT.
Edit: There was a BA PE flight to SIN and back as well that I missed noting above.

, so I guess rather than saying 'below J' I should have said 'not Y'.