This thread touches on a lot of issues that have come up repeatedly over the last couple of years.
There are a few different issues here, I'll attempt to address (add my 2c) some:
Currently the 3D models supplied in the official libs are provided in the kicad-library repository. Note that this is separate from where the footprint files are stored - these are the individual .pretty repositories.
There have been suggestions that the 3D models should be stored with the footprints they are associated with e.g. https://github.com/KiCad/kicad-library/issues/162 - I am in strongly in favour of this.
The current approach means that the schematic symbols library grows large with files that are not associated with the schematics. Adding STEP files which are nominally much larger than the WRL files they would replace (or complement) would only add to this.
Separating the 3D models out would at least allow users to select which footprint/3D repos they download.
By my count, the kicad-library repo (inc. 3D models) is just over 1GB. The sum total of all .pretty repos is under 100MB.
This must be addressed by both the lib team and the dev team.
Converting WRL files to STEP files is not a good idea. Many of the recent 3D file additions have been parametrically generated (FreeCAD) which is great. However a lot of models in the official libraries would need to be completely replaced.
Any models added to the libraries would need to be checked in terms of copyright - many companies do not allow redistribution of their 3D model data.
In the mean time, we gratefully accept any contributions of good-quality 3D models. Currently the best place to submit them is kicad-library