A lot of this is true, but he's already building tons of his own systems from scratch. For an engine that was never made for the systems he's putting into it. He's doing shit the codebase itself was never intended for and its bursting at the seams.
Switching to a proper commercial engine might be a similar difficulty, but it'll cooperate with him a lot more. If he finds a dedicated coder for it, like I mentioned in the same post, it'd probably be less trouble in the long run than the jank way he's doing things now.
Also, I really only mentioned Unity by name just to get the name of some non-proprietary engine into that sentence.