sus·tain (s-stn) tr.v. sus·tained, sus·tain·ing, sus·tains 1. To keep in existence; maintain. 2. To supply with necessities or nourishment; provide for. 3. To support from below; keep from falling or sinking; prop. 4. To support the spirits, vitality, or resolution of; encourage.
No where in that definition does it say that it has to fill a void. What void does the 17 fill while we're at it?
Sustainability has to do with the 20 other calibers that use a 30 Cal bullet and the billions of 5.56 brass already in existence. Unless you expect the 30 Cal market to dry up, 300BLK will be here for a long time. Tell me, when the 17 flavor is over and no more ammo is on the shelves, how do you plan on shooting your rifle? You handload rim fire?
Average Joe's just don't go shoot up a lot of centerfire ammo out of $1000 dollar army guns.
Well, that's just not true, because they do. It also comes in a bolt action and a single shot in case you cannot afford it.