(3) There's a way to completely avoid the previous two hazards, by doing the visual overhaul within the context of an alternative frontend. That way, SMF is left (just about) as-is, with only minor changes to it every once in a while, but there'd be another frontend where we could basically go nuts and try all sorts of fun stuff (without worrying about either of my previous two points). In fact, I'm pretty certain that I could make it so that switching (in either direction) between the legacy frontend and the modern one would be as simple as clicking a button. The problem with that approach is that there's a large amount of technical groundwork to be laid before it would become feasible (and, though I'm willing to do that work myself, it currently falls outside my purview).

In summary: It's harder than it looks, it's probably not a good idea to begin with, and the work necessary to make it easy and sensible hasn't been done yet.

P.S. Don't be discouraged by my answer, the work you did is really cool, it's just that, when looked at from many angles, it's a very small part of the puzzle.