I write software for a living, well as part of my work. One aspect of the lockdown is that it's given me the peace and quiet at home to get on with some big software projects that have been on my back burner for many years.
The problem with major software is the complexity, timescale, and cost. Over time there is another one which is evolution. This might be due to scope creep, fresh ideas, taking advantage of new tech, fixing design mistakes of the past, need for bigger scale, lots of reasons.
There's a couple of decades already of elapsed time and probably thousands of man-hours gone into Survex, Aven, Therion which is the main platform for cave surveying. The program that already turns cave plans into elevations is Aven, and Therion can do that too. We are where we are on cave software, and it won't evolve quickly from here given that the developers are highly skilled volunteers.
It's a mistake to confuse "below a certain level", in other words a Z-axis value, with a map layer. The former is absolute separation of distance while the latter is a "logical" separation of entities. So a slicer won't work for cave surveys as it is cutting on the basis of a "knife" moving along an axis like a ham-slicing machine in the butchers does whereas what we want is separation of one (or several) passage series from another one (or many) on the basis most likely of speleogenesis - and thus their "logical" part of the complete cave rather than as absolute coordinates. Imagine what a slicer would do to the 3d-maze that is OFD2? The slices would be interesting images and a technical triumph but not useful in caving terms.
In terms of the practicalities in getting the detailed quality Draenen Survey properly published (the subject of this thread), it is going to be easier but not necessarily quicker, I feel, to fix the PDCMG than develop a new cave survey software suite. The software might take a decade or more, while the politics probably the same sort of timescale given the experience with PDCMG to date - but much less if a few fresh faces come forward to turn the page.