To be fair, he should have known that it wasn't going to be doable in that timeframe; a rewrite of that scale is something that even us in industry won't approach without serious planning and consideration. But yeah, I don't doubt for a second that the frustrating tangle is a part of why he gets demotivated and prefers side projects.
Ehh, I'm not an expert on ToT's codebase in particular so take it with a grain of salt, but it's salvageable, a 4 maybe. The issue is that it would take a long while and it needs to be done by someone who's simultaneously a good scripter with RPGM and knowledgeable about how ToT works right now. It's just a pain in the arse, and Koda can't be doing a spring clean when he's meant to be doing 7F and a hundred other things. I'd put my hand up for it, except I have a job as an engineer and I don't have the ToT knowledge yet anyway. Not to mention that it would break/disrupt their work. It's a mess; imo they need a sequel game where they can use MV or something, but then they have to learn how it works lol.