I'd be willing to bet the most annoying thing the gov't sees with that statistic is $51 million in lost tax dollars.
The impact is definitely larger though than just this though. A quick look on gunbroker and the amount of used uppers and barrels with < 16" is more than I've ever seen. Used to be it was almost impossible to find a used MPX/MCX barrel/upper under <16" and they are full of them now. Same for both of those being sold used as pistols, gunbroker is flooded both with people trying to dump used ones and dealers trying to dump new ones.
Also more long term how much does this hurt the gun industry through loss of sales, not just of braces, but how many people bought braced guns the last 10 years to get around the SBR issue, that will never buy another gun of that style because they won't do the SBR path. That's a lot of gun sales, brace sales, and all the accessories that go with having another/more guns lost.
Let's think about that, lets say the 30 million braced guns sold/made in 10 years is accurate, that means the gun industry can expect to lose 3 million sales per year (granted some might use that $ to just buy other guns) that's a lot. When you consider a record gun sale year is ~20 million total gun sales, braced guns would then make up 15% of all gun sales.
Even if you take the lowball estimate the ATF made of say 5 million over 7 years, that's still almost 1 million per year, or 5% of all gun sales. The govt' and ATF might be looking long term that they put a dent in the # of these guns being sold and hurt total gun industry sales/income. Similar to how they've tried to make ammo more expensive/harder to get, to take a more subtle path to hurt the gun industry and gun owners as a whole.