Everyone already shared their fair take about 2D Vtubers source file access rights, and I'd like to add from technical viewpoint that Lina's bring here.
You can, in fact, import moc3 file back into Live2D Cubism. However you can only import them in Animation mode. Which means you can make new animations out of any Vtuber models, and export the motion3 files. That's the most extent the editor allows you.
Anyone can "steal" your physics settings. Physics json delivered in your models is a readable text format. Live2D Editor has function to import physics file. Give someone an hour to recreate the parameters and they get your whole physics setup without the model.
And then anyone theoretically can make expanded use of their models to be used in games or other applications. I'm not sure if this has been tried however. I do heard some Vtubers games featuring Live2D models but I'm not sure if they are using actual models.
Those are list of possibilities of what you can do with Vtubers moc3 files that I can think of.
Now what if Live2D editor allows us to import moc3 files to Editor? Say that, deformers, parameters and meshes can be imported, but read only. Only modifications they allow is either opacity, screen blending or removing the objects. Moc3 has no PSD info attached, but you can regenerate atlas from imported meshes. Any people can add new PSD on top of those and rig them just like they normally would, with a moc3 as a base rig.
I don't really have much knowledge in 3D modeling space so I appreciate if anyone "umm actually" me on this, but with exported 3D models you probably losing info like bone names, rig script, modifiers, but anyone can import their 3D models back to Blender and edit the meshes or even retopo it as they like. We could have same thing with 2D models.
However that means Live2D has to rewrote their 20 years old codebase from the ground up (happy 20th anniversary btw), but it's actionable. The only question if they think it's worth the effort.
Live2D is still in this weird spot whether they want to go all in Vtubers, Game devs or even TV Anime production (yes they are making anime too. Aniplex owns the majority shares of the company). One decision they make will impact the others. Japanese bureaucracy doesn't help either. With that in mind I don't believe it will changes in near future.
Live2D is the ONLY creative ecosystem where artist's only options are either sharing their entire raw working project file, or a black box moc3 that cannot be touched by anyone ever again.
This is NOT normal. This is an artificial situation caused by Live2D being closed.