7.62x39mm Soviet dying out?
Probably not for at least 75 years, and then, it will probably only drop off in a slight curve. The Soviets handed out way too many AK rifles in the Third World for that cartridge to go extinct. It'd be like finding a better material for building houses than wood... sure, it might be BETTER, but for half of the people in the world, wood is just fine for a home, and they aren't going to switch.
Sure, the 7.62 Soviet might not be a sexy, high velocity supermodel like the 5.56x45mm or the 6.5 Grendel, and it definitely lacks the longer range take down ability of those two cartridges, but for the tribesman in Africa, the coke smuggler in Colombia, and the man defending his family in the Middle East, the 7.62x39mm works perfectly.