I see Unity lasting a long time. There are basically only two companies to consider if you want to make a 3D game: Unity and Unreal. Unity is so much easier to pick up for newer game developers, which means Unity will always have a funnel of new users that will help the company stay afloat. The decision to use C# is a major factor in Unity's success. Not only do you get the benefit of an existing ecosystem, and you also don't need to worry about the insanity that is C++. The biggest threat to Unity is probably Godot achieving a Blender-like success story where Godot achieves parity with Unity but is free.