This was an attitude I had for quite a while because of an offhand comment I heard from a guy at Knight’s Armament about 10 years ago. ‘It was designed as a impingement gun, we’re gonna keep building them as impingement guns’.
It might be more accurate to point out that the AR-15 isn’t precisely a direct-impingement gun. It uses an internally captured piston mechanism. Gas from the tube flows into the carrier key, expands in the piston chamber (near the gas rings), and pushes the bolt carrier backwards. The cam pin rotates the bolt while the carrier reverses, and once it unlocks the whole assembly moves directly rearwards as a unit to start extraction and ejection.
Piston-driven AR pattern rifles have been established as more than reliable. The HK416 has been in service since 2004 and is considered one of the most reliable piston ARs on the market.
So it’s not that a piston design is or isn’t inherently superior to the AR-15. The benefits of cleaner running and more efficient use of gas are very significant. But the DI system is much more economical to produce, because the piston design requires better engineering principles and production quality.