MiTTs were also manned, trained and equipped for security force assistance. The problem IMO isn't who is doing the training, it's who is being trained.
I think that the Army is preparing for a problem that it wants to have, rather than the problem it does have. The problems we face in Iraq and Afghanistan and Syria aren't military problems, they are political/ideological/institutional ones. If we want to make a difference there we need to start standing up brigades of economists, lawyers and LEOs, under the aegis of the State Department, not taking away our very limited Army combat brigades to support a TTP that we've seen time and time again simply does not work.
We already have too much on our plate as it is. What's coming off to support the 16 weeks of language training and the 6 weeks of SFAB training and all of the other stuff that they haven't thought of yet but are going to need to train on to make this work? How are they going to do language sustainment? What languages are they even going to train on? And what happens to all of that great training and SFAB experience when people PCS back into the Big Army? Or are we standing up a new SFAB MOS where people stay for their whole careers? And if we're going to do that, maybe we stand up a whole new branch, joint/combined/interagency. Because that's what it's going to take to make this work.
And what's going to happen to these fancy SFABs when some manpower-draining contingency like Iraq crops up in the future (looking at you, North Korea)? We don't have a draft, most of America is too fat, too dumb, or too criminal to join, and no matter how we fight in the future, we're going to need thousands of people on the ground. It would be better to designate one brigade per division as the DRB-S, the Division Ready Brigade-SFAB like you have the DRB for contingencies. That way they can stay sharp on basic soldier skills and prep for the SFAB mission just like they would any other contingency.
Bottom line, our general purpose forces are GPF for a reason. The kind of half-assed over-specialization envisioned for SFABs is not likely to increase lethality, survivability, or mission accomplishment. In fact, it will probably have the opposite effect.
This is a bad idea, already poorly executed.