My take on why MF doesn't get a visual overhaul
... Aside of Riot trying to find a new VA for her. You may want to prepare your downvote cannons regardless.
Here's the thing: MF's prostitute look works very well to get Gangplank off guard. However, I think MF knows that the world is a dangerous place and to get ahead, perhaps it'd be better to keep up the prostitute appearance and looks, until the time is right and then she surprises her enemy with her real, ruthless self. Capt. Fortune can still be canon, but we can say it's like a Super Mode for her. By default, you control her flirtatious self, but if you use her skin, let's just say that it's the equivalent for MF to say "OK, shit gets more serious... a little bit." (because she hasn't gotten a VO update)
I guess that'd be a food for thought and what we'll deal with until we get a VO update for MF (C'mon, Riot, I'd love to. Plus the removals of 'Summoners' in her quotes for consistency's sake)