For the sake of discussion, I will assume that the militia is hopelessly and permanently obsolete.

Stating a rationale for a thing does not imply that there are no other rationales. Stating a rationale for a thing also does not imply that the continuation of the rationale is a necessary condition for the thing's existance.

Suppose my wife married me for my youthful good looks. Suppose, in thirty years or so, I lose those good looks. The marriage wouldn't disappear just because the original reason for it disappeared.

Stating a rationale for a constitutional right does not imply that there are no other rationales. Stating a rationale for a constitutional right also does not imply that the continuation of the rationale is a necessary condition for the right's existance.      

Suppose the 1791 constitution of Lower Slobbovia stated, "The circulation of broadsides being beneficial for the transmission of ballads, freedom of the press shall not be infringed."   What would that mean for modern Lower Slobbovia? It would mean that freedom of the press shall not be infringed. The right wouldn't disappear just because, in Lower Slobbovia, ballads are now transmitted by 8-track tape.

A constitutional right doesn't disappear because one of its rationales disappears. It disappears only if the constitution changes or, if Mr. Jefferson was right about that whole "unalienable rights" thing, it never disappears at all.

Kent