It's in Apple's best interest to continue supporting both. Why? Let's just remove the POV of app developers and simply look inside Apple.

The article mentioned a very important point. Apple itself has not adopted Swift as fast as the developer community. This is because (I don't know this for a fact, but am quite sure), a large portion of hw/sw engineers know how to write C, C++, and Obj-C. They don't just know how to write it, I bet a lot of them are damn good at it and if not, top in the industry. Why convert? As many mentioned in the comments, it would be a huge overhaul and what about Apple's internal engineers? Would they put themselves out of a job?...Not likely.

Maybe in 20 years (if there still is an Apple...), there will mostly be Swift engineers. I just don't see them shooting themselves in the foot by removing obj-c in the near future (as in 5-10 yrs).