I don't see that what Nato uses in anyway impacts the cost to me, the U.S. Taxpayer. As outlined elsewhere in this thread we paid some large amount of money to the Germans to design this rifle for us. (It might have been cheaper to hire Russians and resulted in an even MORE durable rifle. Compare a Mosin Nagant and a Mauser for instance, or a G-3 and and and AK-47.) Anyway, most of NATO won't help us when we need 'em anyway.
I say lets arm the "worlds policemen" with a decent rifle/catridge combination and screw the French and Germans. The Brits are our most reliable allie so partnering with them would be fine. Their current rifle is a total disaster, the weird bullpup thing. So they need a new one too. Other than that, who cares.
NATO: North Atlantic TREATY Organization. As in, created when we signed a treaty, thus making it and its rules "the law of the land". Thus, when it comes to such things as cartriges, we HAVE to all use a NATO-Standard round.
Bottom line is, if WE want to adopt a new one, we have to at least try to get NATO to do so as well. It's "The Law".