Depends on condition, history, 'collectability', etc...
An original M-16 should still be worth some coin just for the history, but it's price should indeed drop substantially.
Firearms that aren't produced anymore, period, should have more value. Indeed, with the market opened up more, they may actually increase in value some due to more people getting into the sport.
For example, stamp collections are currently falling in value because, well, 'nobody' uses stamps anymore. Without the day to day familiarity with them, the interest has plummeted.
More low end ex-NFA weapons will open the joys of NFA ownership to ever more people, and that will stimulate demand for the historical ones. It would be something of a roller-coaster ride before things settled to a new equilibrium though.