Used racks are full of all calibers. I see a lot of 223's, 12g shotguns, 30-06, 7mm mag. You name it. Are they fading out?
And what is fairly cheap? I bet no different than any other caliber. If they are tell me where you are and I will buy the lot. A Buddy of mine bought a (new) 300RUM last weekend. Here they are no different than any other gun in their model.
I'll give you that they have no real advantage under 300yds, but whats the matter of being ready for your buck of a lifetime at 450.