That's the very same reason why the US Army hasn't updated the M16's and M4's gas system. It's a 40 year old design and possibly the worst in the modern firearms industry, and H&K developed the HK416 as an alternative for the US military- however indeed it would cost too much to replace all the existing weapons in the US Military's arsenal. It would even cost too much to develop and implement a modification kit for the M4/M16's gas system.