I think you're overestimating the contraction of the AR market.
One, expect a blip again in sales later this year because of the election. I personally think Remington is being a little hasty in closing up shop now before the election even happens. That's like shooting a functional plow horse with no replacement substitute the winter before plowing needs to be done. That being said, if Remington is competently run, which is isn't, they'd be able to estimate how many guns they'll sell with a Trump victory versus a Dem victory. They can look back at 2004, 2008, 2012, and 2016 and say "ok, we'll probably sell X number of ARs during this election year if Trump happens or Y number of ARs if the Dems win."
Two, this is really only the second major producer (counting Cerberus as one entity) to stop producing ARs. The other one was Colt and Colt's been dying a very slow death. Both Colt and Remington have went bankrupt within the last five years. These are two very weak gun companies. There's no innovation or really compelling reason to buy a DPMS or Bushmaster or a Colt AR. The market is very diverse, ranging from PSA to LMT in regard to price and quality. Colt and Remington are two heavily debt-laden bloated corpses that really offer nothing to the market. Smaller companies like say Windham or YHM don't have that massive debt burden that Remington and Colt do. See
@Knob Creek's link, post No. 20. Cerberus loaded Remington up with debt and then shoved everything into a massive Alabama factory and made Remington foot the bill. Colt is also swimming in debt. Is Windham? Are the other small companies? Not as far as we know.
Colt and Remington have been on death's doorstep for quite some time, so to see either of the two contracting isn't a shocker.