Why would they have to overturn the NFA ? The NFA doesn't give the ATF the discretionary power to create law.
First - let me clearly state - I hope I'm wrong. and, just because I think this - does not mean I want this to be the outcome. (I know if one does not parrot the thoughts of the 2A youtube legal "experts" - one can get hammered on online gun forums. Getting accused of "defending the ATF" and being a "boot-licker", etc. Actually I think this forum in one of the best for not doing that.)
Based on PASSED congressional legislation/definitions, I think it is way easier for an ATF attorney to argue that a braced pistol is an SBR vs a group of attorneys arguing the minutia/technicalities that a braced pistol does not meet original 1934 NFA definitions. I think its easier for ATF to make the argument - "yeah we screwed up for the last 10 years, but our current rule/interpretation is more correct to original congressional definitions for an SBR (1934)......and we waived the $200 as a correction."
In the big picture, I don't think the ATF screwing over folks in non-SBR states/counties and all legal buyers over the past 10 yrs is enough to oveturn it. That 16" barrel limit part of the NFA 1934 will have to be overturned - in my opinion.
I think the ATF Braced pistol argument is based on configuration. They are not making a part illegal. In the new rule Braced pistol configuration is consistent with congressional definitions and the NFA 1934.
I think for the Brace rule to be overturned, parts of the NFA Act of 1934 would have to be overturned. I'm not convinced the courts or SCOTUS is willing to go that far. I hope they will, but I'm skeptical.
This thinking is why I complied and registered 6 braced pistols. Just like I complied when I bought them and didn't add a stock.....or when I fill out a form 4473....or when I bought and completed form 4's for suppressors....or when I check a firearm thru TSA....etc....etc. After the Patriot Act - none of us have any privacy anyway. The Government can find out what i have if they really want to.
If the rule is overturned, there will have to be a process to void Braced pistol approved form 1s (there is no stamp for those). If that happens, I will probably void 2-5 of those registered and just celebrate any free stamps I got.