Direct impingement was already fielded by the French in the MAS rifle. It showed a lot of potential apart from the caliber, and got a lot of attention.
When Stoner adopted the system, several things were brought together that hadn't existed in the same firearm. The barrel extension locked the lugs, not the receiver, which made the stressed parts short and direct. Using multiple locking lugs reduced the stress each had to deal with, rather than concentrating it in just two, and directing the stress through a receiver. By directing the gas into a chamber in the bolt, it dropped the piston, operating rod, and all the external tipping forces, and operates coaxially with the barrel, not apart from it.
The gas directed into the bolt carrier is only enough to open the bolt - the amount of time in the cycle that gas is directed is limited to those fractions of a second that the gas key surrounds the tube. As pressure rises, the bolt carrier moves, the cam pin rotates the bolt, and within an inch of rearward travel, the key is disconnected from the tube and no longer receives gas.
I describe this because thermal measurements of AR 15 bolts show they aren't much higher than piston operated bolts. One study has them only 25 degrees warmer. It's not hard to measure the temps with inexpensive non contact thermometers - which put numbers to the uninformed rumors of the past.
Once the bolt is moving rearward, it compresses a relatively light spring, largely because the carrier itself is light. The bolt carrier of a roller locked HK is a virtual weapon in itself, the AR is a joke in comparison. With the accompanying bolt hold back, a feature many battle rifles didn't have, the AR was easier to keep loaded and firing, as it did not need to be taken from the shoulder to cycle the action. The magazine could be dumped as soon as bolt hold back was perceived by simply using the trigger finger, a mag inserted with the left hand, bolt release by the same hand already in close proximity, and the trigger finger reapplied quickly because it was never removed from the grip.
Name the rifles available that could do all that in 1958. How many still don't?