I seriously doubt the Phantom line will be abandoned by DJI. The Mavic will be a fantastic drone for most people and will become the favorite of travel vloggers and aspiring amateur film makers and hobbyists, but there will still be a demand for the more robust Phantom. Firstly, there's payload - due to the Mavic's small size, weight, and battery capacity, there's a limit to the gimbal and camera that they can install on it. I see DJI releasing a
P4 Pro or perhaps a P5 with the Zenmuse Z3 22-77 zoom lens installed, and maybe even an improved gimbal. No way a Mavic can carry such a setup. Secondly, a Mavic is unlikely to survive even a small crash without significant damage, meanwhile I've seen Phantom 3's and 4's have some spectacular crashes and still able to fly again after replacing nothing more than a few broken props. There will be always be a need for workhorse drones like the Phantom.
That said, I fully intend to make the ultra-portable Mavic my own workhorse drone... until I crash it and have to buy a new one