The M27 (modified 416) has already been chosen to replace about half of the M249's. When the government has hundreds of millions in the M4 and all of their replacement parts, I would not expect them to replace with something else. As I said before it isn't for everyone. IIRC the cost of a complete 416 to the government is $1100 which is almost 2x the cost of an M4( link to the m4 cost per rifle
http://www.fas.org/man/dod-101/sys/land/m16.htm).That does not mean the 416 isn't superior. I trust Delta,and the SEALs, when they say it is a vast improvement and their weapons of choice. They are the best of the very best, and know what they need to do their job. They just don't have the budget woes an average soldie does.