"...450 Marlin rifle has been discontinued and Hornady no longer makes the ammo..." Still listed on Hornady's site. So is the .338 Marlin. Kind of silly to make the .450 on a belted case.
"...hope they at least keep making brass..." Not likely. When cartridges get dropped by the marketing types, it's completely dropped with no regard for their old customers. When you buy any new cartridge, buy as much brass as you can, dies and a shell holder(unless the cartridge uses one in common with an older cartridge) too. Even if you don't reload.