It's a lot more complicated than that. We're talking hundreds of thousands of animations and millions of lines of code. At that point, you might as well just build a new whole game.
Not to undermine your point here (I think I'm actually supporting it?), but, well, they did. FFXIV heyo!
But yeah. FFXI has moved well past the undead zombie of a game phase and straight into the undead immortal vampire suck-your-blood-til-you're-hollow phase. Which is a metaphor that even I am struggling to comprehend.