Should 6.5 become the new US military standard?

Should return to .308 or .30-06, since .223 is merely varmint rifle. 

The 6.5 would be going back to the early 1900s

Think about the millions of rifles and machine guns which would have to be converted or replaced.

Military gets ammo in lots of hundreds of millions and the 6.5 would not justify the expense over .308 (7.62x51) or .223.

6.5 may outperform the .223, but not the 7.62x51. and not that much better.

The 6.5 Creedmore is not that much better than the 308 that it would justify switching over.   It is maybe 5 percent more accurate but the cost of switching would run in the millions.   If anything we should get rid of the 223 and go with the Five Seven for close in work.  

In addition to many of the other good answers, I'll add this:

One of the decisions behind round choice for the military is how much it weighs.  Soldiers have to carry it, so there is a trade-off between more power versus carrying less rounds.  Does the Grendel outperform the 5.56?  Sure, but it weighs noticeably more, so soldiers can carry noticeably less of them.  Sometimes it's less about the terminal performance of the round and more about volume of fire.

I believe 6.5 Creedmoor would be a great cartridge for the military, however, not particularly the 6.5 Grendel. The AR-308 upper/lower would have to be used so the argument of 7.62x51 in AR-10 has already been tried and proven so why change to a not-so-drastic cartridge? 

Also, no one knows where the next confrontation will be...urban, mountains, jungle, or all the mentioned. We have already experienced that our armed forces had too much rifle for the jungles of Vietnam so the M-16 in 5.56 (and variants) proved more compatible than the M-14. I liked the M-14 in 7.62x51, but long since my tour of duty the service personnel have become mixed gender and more recoil sensitive. 

I have recent personal experience with 6.5 Creedmoor and due to the heavier BCG/buffer system the felt recoil is less than one would expect from such a long distance cartridge. The accuracy is superb IMHO and well makes up for the jolt in the shoulder.

Weapons will be banned December 1st. Seizing of them will be December 2nd-23rd

Many moons ago when the Grendel was an Alexander Arms proprietary cartridge it had a bad reputation for cracking lower receivers at the rear take down pin hole. That alone would keep it from being considered for military use since they'd most likely use the same lowers they have now and refit them with Grendel complete uppers. Also, the Grendel is a commie cartridge in a roundabout way, it's based on the 220 Russian, which is based on the 7.62x39, this might not sit well with certain small arms appropriation committee members. 

Doesn't hit as hard or travel as far. That's why. We don't send soldiers out to influence people, we send them out to kill.