In my opinion, I feel like 9mm would be the #1 caliber that isn't going anywhere, any time soon. Along with 9mm, I'd also throw in .357 mag, .40 S&W, .45 ACP, .38 Spl, and .380 ACP. I feel that calibers like .25 ACP and .32 ACP have the highest chance of completely being phased out. Namely the .25 ACP. I feel .22LR has it's place as the "smallest" caliber for easy shooting. It's solidified it's popularity in this regard.