Eevee is far from being a replacement to Cycles, and may never replace it entirely. It most likely will see close integration with the Blender game engine, opening the door for a much more robust game development environment inside Blender. This puts it more in competition with other real-time game engines like Unreal Engine or Unity. We may see some tandem functionality between Eevee and Cycles down the road, allowing for some very interesting interactivity and unexpected real-time results. It's currently running only on CPU which is not very good at all for any real time engine. Once it gets ported to GPU then we will see things get much more interesting and useful.

Blender Internal is probably only still around at this point for backward compatibility purposes. Then again, Autodesk has never removed the old Scanline render engine from 3dsmax either, even though the software is now shipping with the powerful Arnold render engine by default, and also has no less than 3 other default render engines built in as well.

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