True full-auto guns are not illegal (at least per the Federal government. States have their own laws that may preempt Federal law). Citizens may own a transferable full auto on a BATFE Form 4 after paying a $200 excise tax and passing a background check, complete with fingerprinting. Statistically, such legal, civilian owned guns in the hands of their owners have almost never been used in crimes. In fact, the only case ever cited since the National Firearms Act of 1934 was committed by a police officer who used his legally owned SMG to rob a drug dealer. There may have been other cases, but nobody seems to be able to document one.
Simply put they have not ever been a threat to anyone.
Since GCA '68 and later the May 1986 legislation no further full auto guns may be imported or manufactured (i.e.; legally converted) for civilian use. So the supply is static and the prices are beyond the reach of anyone but the rich cronies of the gun-control elite. This legislation was passed only because some politicians found it to be any easy mark to hit, and people were scared of them.
You are correct in referring to the Slide Fire and other bumpfire toys as "virtual" full auto. And yes, I'll bet they get pushed into the NFA corner soon, especially after some psycho looking for attention commits an offense with one. it will only take one such criminal and one such offense to cause this.
They are toys, and are meant only to convert money into noise. We should be pretty safe from the effect of screwball bump fire legislation until they outlaw rubber bands.
Spend the $350 bucks you would dump on a bump fire stock on ammo and then practice bump fire until you learn it.
As far as arguments go in favor of them being legal, just show people the various You Tube videos on the subject until it sinks in that you don't even really need them. They just help somebody to learn to bump fire sooner and with better accuracy.