I think you are confused, as usual.
Yes, things are adapted over time.
SO what has changed over time?
1) Militias have been replaced by professional police forces and the National Guard.
2) Guns have become a lot cheaper, to the point that anyone can afford them, even people who have no business having them.
3) The guns have become a lot deadlier since 1787.
So instead of saying, "Well, the Founding Slave Owners said we should have guns because of Militias", we should ask, "Do we really want a semi-automatic weapon in the hands of someone like Adam Lanza or Awake the Rapper?"