Re: mid-sized to smaller markets: The time has already come on that. Pre-recession there were a number of stations in the 50-100 market range flying choppers (just look at late 90s/early 2000s news opens -- a LOT of stations had them). Now, to my knowledge, there's only KOTV in Tulsa, and the ultimate outlier, market 140-something WBOC. You can rest assured that their choppers would be grounded if either got sold.
As for drones, they are absolutely not a replacement for choppers, regardless of how companies will try to spin it. Try getting a drone to an industrial fire clear across town in 5 minutes. Not gonna happen. At the same time, stations probably aren't sending their choppers up much to get aerials for a non-breaking-story. The two pieces of technology largely don't overlap IMO. Drones can be used for breaking news, but they can only get to scenes as fast as everyone else can driving, they take a while to set up once at the scene, and the operator has to hope that it's in a zone they're able to fly in and officials haven't put up any drone flight restrictions. Also, unless it happens to be near your station, you're probably getting more of the aftermath than the event itself. Choppers don't have the same restrictions over where they can fly, when, and how. You can fly a chopper over a crowd of people all day. Forget about a drone doing that.
Basically, companies getting rid of choppers have decided the juice isn't worth the squeeze, and that whatever they're gaining in terms of spot news coverage doesn't outweigh the enormous amount they pay to fly the thing. A drone may soften the blow slightly, but I think it's a move these companies would be making whether drones existed for news or not. Having a drone just softens the blow a bit.
I don't think you'll see Top 10-15 stations dump choppers anytime soon. They're still getting their money's worth. I'd consider them endangered in lower markets, especially the 30-50 range.