Just some random observations
Grain fed or grain-finished beef I understand is better for (dry) aging. The beef-y taste of Argentine style barbeque (grilling) is of course from the grass-fed beef, but also the style arises from grilling soon after slaughter.
Grain fed beef is an American institution, but to me it makes sense that the increased fat and marbling, leads to moist and good texture in
aged steaks. In this context, of course the aging process would concentrate beef-y flavor of the meat, while the high saturated fat content (especially if well marbled) would help deliver that (though the more fat that prevents dehydration, the longer one has to age). The taste of that fat would surely also depend on the diet (therefore what's in the grain and other food). Again, given that most beef (and meat) sold in this country sits in a refrigerator for considerable lengths of time (versus say a fresh carcass from the corner butcher's), it is not surprising that the industry and (shaped) consumer tastes are at the current point.
Uh, I'm not sure where I'm going with this, just thinking aloud I guess. At any rate, while I prefer the taste of grass-fed beef,
aged grass-fed beef makes no sense to me.