[You can make some terrific sauces with marrow; Bordelaise sauce for meat, suga sauce for pasta and others. Younger generations reject them, in the same way they reject organ meat, tongue and the like.
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At least around the Northwest here, there's been a bit of resurgence in popularity of marrow bones amongst the younger crowd. I've been to a couple of restaurants in Portland that have marrow bones on the appetizer menu (and a few more that have added osso buco to their dinner selections). A few of the larger Asian markets that carry bones, but definitely not standard stock for 'regular' stores.
As for organ meats, I am one lucky girl to have grown up eating all that good stuff! There are a few Mexican places around that have 'lengua' on the menu, and thankfully a few groceries carry beef tongue around here too. I've introduced a lot of friends to organ meats (we raise our own chickens and rabbits), and they are always surprised with how delicious they are, since they didn't grow up eating it. My 14 year old step son hasn't taken the mental 'plunge' for tongue yet, but sure eats his share of sauteed rabbit liver!