So why is AK credited as such a breakthrough in design?
The AK wasn't a breakthrough in design. The German Sturmgewehr 44, adopted by Germany in the closing months of the war in 1944, was the first true "assault rifle" using select fire (semi or full auto,) and a medium power cartridge that was more powerful than the pistol cartridge based submachine guns of the day, yet more controllable than the full sized rifle cartridges of the day, such as the .30-06 chambered Browning Automatic Rifle.
Where the AK shone was in construction using many stamped steel parts that could be built very inexpensively, and very loose tolerances among the moving parts which let the rifle operate for an extremely long time without cleaning. The tradeoff was less accuracy.
Plus, so many of them were built by the Soviets and handed out worldwide in what was called "AK Diplomacy" that its sheer numbers ensured it would be a significant rifle in history.