Jim, I wasn't saying every ski under 75mm is race oriented or race derived, I was saying seeking out narrower, high performance hard snow skis is as much a niche purchase as buying a wider ski for soft snow. I acknowledge that there are thinner skis that still work under the 'mid fat' banner.
Secondly, though...why are short carved turns the most desirable turn for hard snow and groomers?? Even on a hard snow day, one of my favorite things to do is link together the longest possible large radius turns I can, using every bit of available slope. I love skis that excel at this sort of turn, In fact last season, I made a decision to pretty much eschew short turns except when absolutely necessary and explore the world of big, space-eating carved turns (and huge drifted turns as well) whenever skiing open slopes, groomers, and even on powder days (The big sweeping, terrain-gobbling powder turn is one of the most commanding feelings you can generate on skis IMHO).
My friend who taught me to ski pointed this out to me a few years ago, and if you look around you'll see it's very true, but on most slopes, especially eastern one, most people lock into a single relatively narrow corridor and ski it the same way the whole way down a mountain. This is true even of very good as well as very not good skiers. I notice it even skiing midweek on wide open cruising trails, folks tend to tunnel vision and short turn (with varying level of ability) down the same relatively narrow slot missing out on the ample amounts of delightfully open area to fill with your turns. Of course there are also those who most ski straight down the hill due to lack of skill or concern.
For years I skied this way as well and owned a number of SL type skis to serve that purpose---but I've really changed that now, and now even for my mid range ski, I want a tool that excels at medium to bigger turns, even on hard snow and groomers.
I think the prevalence for the all purpose, mid fat with the shortish TR is part of this phenomenon.