Am I missing something? Are you saying that the .30 Remington AR -- a cartridge introduced in something like 2008 -- is a decent, old cartridge that has been pushed off the ammo shelves? That doesn't strike me as old. It seems more like a new offering that just didn't catch on. While I was admittedly never looking for it, I suspect it was never on a lot of ammo shelves anyway.
Fair enough on your larger point, though. That is just the market doing its thing. Right or wrong, some old things remain relevant in the marketplace and other things do not. I think we still want innovation in our marketplaces though. ... and adding one offering while taking away another still leaves us with at least as many options to choose from.
Some of the old good stuff does not get shelf space. Some of that is regional. I work at a Midwestern outdoor retailer. I see guys looking for 8mm and 7mm Mauser, .257 Roberts, Various Weatherby stuff, .35 Remington...and a ton of older cartridges that do not get much support from ammunition companies. The various newer chamberings are there but not the older stuff. The tried and true like .30-06, .300 WM, .308, .270, .30-30... But if a guy has something that was once considered a deer camp standard then good luck.
Some chamberings that were a flash in the pan like the .30 Remington AR came and went. Hunters and shooters may have bought rifles thinking they would be set for many years to come are stuck with tomato stakes unless they reload and can find brass. Other newer "next great things" have replaced the older "next great things". The standards just keep chugging along. .30-30, .308, .300 WM, .30-06, .270... What can't you do with that lineup? Why do we need anything else?
My gun safe includes .22 LR, .22 WMR, .380, 9mm, .45 ACP, .223/5.56, .30-30, .303 British, .243, .30-06, .300 WSM, 12 gauge, 20 Gauge, and .410. The newest chambering I have is the .300 WSM and that is a 24 year old chambering now. I will not hate on anyone shooting something new. For me I can do everything I need to with my line up.