Historically, the 2nd Amendment exists because of the deep distrust of standing armies in the early republic, yet something was needed to defend the country and the frontier. The militia system was intended to remove the need for a standing army. So if we really want to return to our Constitutional roots, we need to get rid of the U.S. Army, or at least reduce it down to a single brigade.
Sure, in theory. But I can almost guarantee you that if Madison were alive today, and had adapted to the modern world, he would not support repealing the Second Amendment.
So he'd be in favor of making major cuts to a bloated military-industrial complex? The idea that we mind read people dead two centuries is ludicrous.