I had a chance to talk with a decorated Green Beret over drinks about this over the weekend. He was thinking that the SF teams will try to keep the M4 variants they use. He said that the consensus was that faster, more accurate follow up shots are more beneficial than what benefit the bigger, heavier platform might provide.
Kind of like the same argument you tend to get for the 9mm over heavier calibers.
Makes sense to me, especially since their SF grandads were the ones who tried out the original AR carbines in Vietnam.