From my experience with gstreamer (at least on the C++ dev side), it was quite a hustle to setup the prerequisites on all major platforms. I guess it makes sense to ship gstreamer bundled, because people using Processing (not library devs) are usually not happy to install any more dependencies.
To be honest, most of the time I am not using most of the gstreamer features, but just a simple pipeline for capturing a webcam. Why do we need the full gstreamer collection for that? Thats why in my opinion it makes sense to have a very simple webcam library, which can be used in these “simple” project.
If Samuel knows Processing? 
But I know the FrameGrabber structure from implementing the RealSense2 binding for it. And I already wrote something similar for a project where I used Processing & JavaCV together.
The question is more, if it makes sense to replace gstreamer with opencv (which is quite bloated as well). It may make more sense to create a binding to webcam-capture.