But the DHS didn't suddenly come about because our military is no longer concerned with defending within our borders.

Actually I disagree with you here - to a point. Since the chance of any sovereign nation actually mounting a full scale invasion of the USA is virtually none, and the existing laws effectively prevent the military from acting within the US borders unless such an event happens, I don't think that military planners do spend much time planning for in-border defense. I think that this was amply demonstrated on Sept 11, 2001 when hijackers managed to fly 2 relatively slow and un-maneoverable jumbos all the way from Boston to NY without being intercepted by fighter jets. And another one into the US Military HQ. Now you could say that this was an unexpected move, or question what action any fighter jet could have taken, but I think the point is still valid. The military failed to have any defense for an in-border attack. And air defense is possibly their most important role, given the lack of credible ground/sea -based aggressors.

But, all of the alphabet organisations exist only because the military's role in domestic affairs is limited by law. I don't think that we'd like to see;
INS replaced by the Army.
FBI replaced by the Army.
Local police replaced by the Army.
State Troopers replaced by the Army.
etc.

That being said, when you look at the specialist police teams, such as anti-riot squads, SWAT teams etc, their munitions and training are such that I don't think there would be much difference if they *were* replaced by Army teams.