You say it was forgotten for a century as if people stopped buying guns for 100 years because there was a standing army. If the government wanted to get rid of it in favor of a standing army they would have done that, or at least tried. There’s no indication it’s a failed amendment because the US needed an army too. And in fact if it was a failed amendment, where was the uproar at the time? I don’t really recall any historical fight against that amendment.