Yep, this is a good question that I’ve been considering myself.
As this is a commercial plugin, I would like to not rely on core functionality from another plugin that could potentially change significantly in the future. Although initially there will be quite a bit of core implementation overlap, I think this is the way to go for long term stability for the users.
I have evaluated the code supplied by Paper2D and I would need to rewrite parts of how the internals work to achieve some of our feature goals. This creates an issue where I would need to push functionality into Paper2D to move forwards with the features I need to implement, but still leave it in a usable state without this plugin.
This will be a bit of a maintenance headache as I will not have control over when Paper2D functionality is updated or what could change underneath me.
Hope that makes sense!