About volume objects, I think it is long overdue. I worked on one back in the days, Brecht also made some fundamental work in this area.
Although level sets could be sculpted directly (and Blender could have a volume sculpt mode if it had a volume object), generally speaking, volumes are inherently procedural and it is best to have a node system to build or process them. The modifier stack is far too limiting here, and currently it seems people are trying to cram as much OpenVDB/volume functionalities into a single modifier, which does not please me. Perhaps scattering the functionalities across modifiers could be better, more acceptable for a merge in master (like putting the boolean operations in the boolean modifier).
IMHO, nodes are the go to for such volumetric based workflow, but unfortunately the current “Everything Nodes” are not suited for the task as their abstraction will not allow for volume to mesh operations to occur right in the middle of the graph. It could allow to modify voxels or vertices in parallel, but not heavy topological changes to the object being treated (this was the same issue with Lukas Toenne’s system for those who remember it). So, let’s not put too many “random” functionalities in one modifier, since a more capable node system will not be here for a long time.
As for C4D, it seems to me that it is a node system in disguise : it is using a hierarchy of objects to express the final result, which is basically what nodes are (think of node connections as being parent/child relationships). It seems like a halfway point between a modifier stack and full blown nodes.