I feel like there ought to be a really good open source game engine competing with Unity and Unreal, but I’m unfamiliar with any.
There’s quite a few. The biggest one right now is Godot. There’s still MonoGame, Open3D, Unreal, and others. Unity has one thing these others don’t (outside of Unreal) and that’s a healthy asset store. Solo indie devs aren’t necessarily the best artists. We know the math, but lack the eye.
I do have the eye, but lack the skills in my hand. I can do shiny art, but am so slow compared to an skilled artist. So why should I do their jobs?
Blender has also an asset store btw, but not really comparible (yet). But the blender foundation would be a player I would trust to invest in them. I always stayed away from unity and it seems my gut feeling was right.