Bump stocks are the Gun equivalent of synthetic legal drugs (think bath salts) that the DEA hasn't outlawed yet. Once bath salts were rescheduled, it did not lead to prohibition of whisky. Bunpstocks did not exist until a few years ago, and they do something categorically different than other firearms accessories. I'm skeptical that it's the slippery slope into disarmament you make it out to be. Bump stocks aren't on the same slope, they're more on a slope with oil filter silencer adapters.
To be clear, NRA never suggested that bumpstocks are destructive devices. They simply suggested that ATF do its job, like it should have in the first place (when first bumpstock was sent in for approval). Bump stocks and even Pistol braces are such absurd interpretations of the law, I don't see how one could defend their apparent legality with a straight face. (Not saying I support NFA, but come on, these are obviously workarounds that play on a Sophist's reading of the law).
I'm afraid uncompromising, unreasonable zealots ultimately pose a far greater threat to the long term viability of second amendment rights. Makes all gun owners look like nutbags and alienates the middle majority we so desperately need the support of.