The Metcalf piece argued that "well regulated" cannot be disregarded in interpreting the described right simply because the phrase "shall not be infringed" appears later in the amendment.
If what is ideally "well regulated" is a population the existence of which does not narrow or define the right described, noting that it does not narrow or define the right described would appear to be on topic. I do not note that to be gratuitously argumentative; I do not understand how that is not directly responsive to Metcalf's rationale.
As was noted above, we do have laws that pertain to our rights, so the right need not be unlimited or absolute. We have laws about how and when we vote, and who may vote; that does not suggest that we have infringed the right to vote.
However, Metcalf's in for a penny in for a pound surrender to the concept of regulation in the modern sense is what does, and what should, raise our civil rights hackles. While we have laws that pertain to voting, we would not stand for a law that allowed voting only if a fellow were wearing a blue tie on the basis that once we have laws regulating voting, an objection to the concept of regulating voting is baseless. We would object to that law as a substantial interference with one's right to vote.
Similarly, the objection to Metcalf's conceptual surrender is that it retains no bright line protection against substantial interference with the right. Why is that a conceptual surrender? Because something that can be exercised only when it has the kind of widespread social acceptance that permits it to be exercised in the shadow of balancing tests and political consensus is no real right at all.