If the libs wanted to lower the potential of stolen AR15 SBRs getting out in the wild, effectively getting them "off the street" like they promised, all they'd have to do is make the AR15 non-restricted with 18.5" or longer barrels. Almost everyone would switch to 18.5"+ barrels, classic 20" Colts would skyrocket in value, and loose barrels under 18.5" would be all over the place for bargain basement prices and seen as almost worthless. Retailers would literally cease stocking them.
If they REALLY wanted to get rid of SBR's they could have a voluntary buy-back for stripped AR barrel under 18.5", say $100 a piece, and they would virtually all disappear from society.
Now that is actually a liberal/progressive move I could get on side with. I see no reason for the AR to be treated differently than any other semi-auto black rifle.