As a coder myself I can confirm that copy-pasting code from an old architecture to a new one actually introduces more bugs than if you had just rewritten everything yourself. Which is probably what the very small dev team is actually doing.
Not to mention that this game is ALOT more complicated then people seem to think.
Has anyone ever played ARK: Survival Evolved? During character creation in that game, one can move sliders, similar to APB, and change the width and height of many body parts. However, one thing that APB doesn't do is cause odd distortion when the sliders are changed. ARK does.
This isn't just luck, APB had to be very well thought out in order to ensure a lack of distortion. Now add this same system to nearly every single piece of clothing, and on top of that add a layering system to the clothing. The clothing underneath has to be scaled to fit under the clothing above without clipping. NOW add decals on top of everything and make sure THEY don't get distorted.
All of this is fine and dandy, but now make sure that the changes don't mess with animations. Now optimise that whole mess, and make it understandable in the future.
Just because a game is old doesn't mean it isn't complicated.