Ammo will most likely remain available, as even extremely old cartridges which have been completely depreciated remain in production, not to mention cartridges which more or less completely flopped like 9x18 Ultra are still produced in limited quantities, such is the beauty of metallic cartridges, they're easy enough to produce that companies will continue to produce ammo for practically any cartridge which will sell, even in the smallest of quantities.
For example, .38 Super Automatic and especially .45 Super Automatic, two cartridges which have never been mainstream, nor have many firearms been chambered in them, yet both remain in production, and can be purchased, just don't expect to find them on a shelf in your average gunshop.
Likewise, even if .30 Super Carry fails to catch on, it will most likely remain in production, and will be available, even if only via special order.
Honestly, the only cartridges I'm aware of which are no longer in production are really old proprietary cartridges which were quickly depreciated like 7.62x25 Borchardt or 9mm Mars.
Other than that, the only ammunition which has ever gone completely out of production in the short term is unusual, proprietary ammunition like Volcanic Rocket Ball, Gyrojet, and Dardick Trounds.
Just one of the many reasons why I laugh at the asserted "death" of the .40 S&W cartridge. Just because something isn't popular anymore doesn't mean it's dead.