I think Biden and his crew have a couple of things going against them. The biggest is the fact that in 1994, The Treasury Department set a precedence about re-classifying weapons on to the NFA. The Secretary of the Treasury designated the "Street sweeper" and USAS 12 shotguns as Destructive Devices, and ordered them to be placed on the NFA. To accomplish that, they allowed owners of those weapons to apply to the NFA for tax stamps, but were not charged the $200.00. So if suddenly the ATF classifies all pistols with braces as SBR's they will almost assuredly have to let us do that free of charge. The changing of the barrel approach is OK for some weapons system, but others will be problematic at best, and cause a true financial hardship on law abiding citizens who legally purchased these weapons in their current state.
The other problem ATF has in doing this is political backlash. Within days of this proposed ruling coming out, 48 Senators signed a letter to ATF to reconsider the fact they may be instantly turning law abiding citizens into felons with the stroke of a pen. And that it could severely impact any future budgets ATF may be looking for. Granted, not a majority in the Senate, but I believe that if this was railroaded on the American people, there will be Democrat Senators that would join with Republicans to make life very hard for ATF.
Again, these are strictly my observations. We'll all just have to wait and see.