The firearms world changes slowly, as, at least with projectile weapons, there are only so many ways to design the weapon/bullet until you have a combination good enough that you see no need to improve it.
The 45-70 cartridge is over 140 years old. It is still the no 1 round for close range hunting in North America.
The 30-06 is over 100 years old. It served as a military round through 2 World Wars, Korea, Vietnam, and even saw limited use in Afghanistan and Iraq, in addition to being a rifle cartridge you can buy anywhere, in any sporting or gun shop, on Earth, as it is an insanely popular hunting round.
The .45 ACP round, and the pistol designed around it, the 1911, have just reached their 100th birthday and show no signs of fading away. Other cartridges, such as 9mm Parabellum, 7.62 x 51, and others, will likely be around for at least another hundred years, as simply, there is no need to look for something "better". They do their job just fine.
Firearm design changes almost as slowly as cartridge design, due to factors ranging from people not wanting things messed with, to the problems in teaching arms drill to an army when you change the model of service weapon. The "bullpup" design, the most noticable change in firearm design for the last century, still hasnt quite caught on. Belt fed machine guns are still the norm for support, bolt-action rifles for snipers, semi or selective rifles fed from a box magazine for infantry.
Look at the changes in aircraft since the Vietnam war... then consider that, US forces anyway, are still using the same service rifle, albeit with some (very necessary) improvements over the original design. The AK-47 and its countless derivatives have been around for over 60 years, and I assure you, they will be around for at least another century, quite likely more.
The stuff works, is reliable, it is easy to manufacture, it is easy to train someone to use. This matters a whole hell of a lot more than being the most "advanced, high tech, lethal weapon in existence!"