The reason that a well-regulated militia appealed to the founding fathers was that they hoped it would eliminate the need for a standing army.
If you had the whole body of young adult men armed and trained, it was hoped you could raise an army very quickly in case of need. If that were true, Congress could insist that the standing army be run down to just a cadre of specialists and officers. Then, in the event that you needed an army—because you were invaded, or needed to invade someone else—you could mobilize (i.e. draft) the militia to fill the ranks with soldiers.
The founders knew perfectly well that a government with a standing army could not be resisted by the people, even if the people were armed. A standing army was always going to be more disciplined, more highly trained, and better equipped than a militia could possibly be.
So, the purpose of the militia was to eliminate the need for a standing army. If you could make a militia work that way—quickly go from a bare cadre to a fully mobilized army simply by calling up the militia—then you’d be in a position where Congress could insist that the the army in fact be a bare cadre, meaning that neither presidents nor generals could use the army until Congress actually raised one.
It’s an appealing idea, particularly to someone like me who is very doubtful about trusting presidents or generals. Sadly, the evidence is pretty good that it doesn’t work.
This was clear even before the sorts of modern, high-tech weapons and other equipment that take extensive training to learn how to use, and then nearly constant on-going training to preserve the capability. It took five years to go from a 16,000-man army to a 1,000,000-man army during the civil war. The ramp up for WW I was quicker but also smaller—manpower grew by 16 times in two years rather than 62 times in five years, but that was from a much larger base—basically, a smallish standing army, not merely a cadre waiting to be filled out.
The experience of the U.S., at least as far back as the civil war, is that fielding an army by mobilizing a militia simply can’t happen fast enough to respond to an enemy with a standing army. (The experiences of Switzerland mobilizing to resist a possible Nazi invasion and Finland mobilizing against the Soviet Union at around the same time are interesting, but do not I think make the contrary case.)
Given all that, I’d have to say that a militia is pretty much obsolete, and has been for a couple of centuries at least.