Realistically speaking, how hard is it to vibe code an engine these days? Unreal is source available and I am willing to bet the source code has been used to train AI models. And there are genuine open source projects like Godot that can be used as a foundation, license permitting (or not). The bigger moat seems to be all the tooling around the actual engine.

I didn't try that hard but I did not have much success. I spent some time trying to vibe code a forward clustered renderer in vulkan and I couldn't manage to get anything I was too happy with. Mostly just regurgitation of a few different tutorials. It's possible I'm just too dumb to use AI and it was also 18 months ago, so things have progressed on the LLM front.