The reason to strike a death blow to the Big East is that, like you said, if things stay as they are now, it will still be a power basketball conference. It will remain on equal footing or a half step below the new ACC and will still control much of the northeast AND have MSG locked up. To me, that makes the Big East a tremendous threat to the ACC and hurts Syracuse recruiting. By grabbing Uconn, the ACC can in one fell swoop knock out it's only rival on the east coast, cause more teams to have to move around thus forcing ND closer to a decision point, and potentially wipe out ND's cushy home for basketball and olympic sports - again putting pressure on them to make a move.
Further, if the Big East reacts by grabbing more teams - they could come from the Big 12 which may make Texas available to the ACC.
I just don't see a downside to adding Uconn. If you add them and the swing and miss on ND, then on Texas, you'll still have your pick of Rutgers or Louisville. As an added bonus it gets us our modern era basketball rival, makes the ACC unquestioned #1 in BB and shifts the center of gravity of the conference a little more north which helps us as well.