>>108129449I know this is bait, but as an early adopter model artist (before Live2D was out in English; before niji EN), I want to explain to anyone that might legitimately not know why the rigging has stayed the same.
First, this was likely a fast turnaround return. Niji knew they were getting a sweet deal by signing her again, and likely weren't willing to waste it. This leaves no time to re-rig.
Secondly, for those that don't know, the fundamentals of how you draw and cut a model have completely changed over the years. The tools used for rigging are entirely different - Live2D used to operate on a parts based system, where a PSD would be a single arm, then another for a leg, so on and so forth. These would later be combined. Around the advent of 2D vtubers, this system had began to change, and there were many growing pains. You could not use clipping layers, layers had to be kept to a minimum, and moving parts (like the hair and eye physics we're used to) were nearly unheard of. You could not mesh a face to move to the side correctly. A lot of people even combined the nose into the skin of the face to save on layers.
It is likely that her model art will require extensive editing and new art to bring it up to date before it can be rerigged. There is every chance they will just replace it entirely, akin to irys (who had similar issues with her initial model). She may even choose not to have it redrawn and rerigged so there aren't any discrepancies between a new model version and her other outfits.
If you'd like another example of a supposedly new model that I believe to have been drawn for and rigged with a much older version of Live2D, look at Ashkariel from FSP. His model has minimal moving parts and pops between expression pieces, just like the initial parts system required.