I am too far from this context to deeply understand all the motivation behind the manufacturers choice. (I just know that open source in same context improved the advantages over competitors)
Anyway I am not sure if my comments in this post are useful (if not, just say, it is not a problem), so I will try to add the last idea:
Would it be possible to collect a list of parts from open source libraries or anyway patented with a compatible license? For example I just found this http://espice.ugr.es/espice/src/modelos_subckt/spice_complete/ from the university of Granada which, if usable, is already quite a good starting point.
Once a kicad library will be created and shipped with the program when the user is in eeschema and try to add a component it would be perfect if an icon appear near it, meaning that that part is already present in the kicad-base-library (and so simulation-ready without any hustle of googling modules buried deep inside old forums). A concept of what I am talking about:
Then the Spice Model Editor should be already filled with all the needed things like file path to the library and model (and I don’t know if more is needed)
If at this point of history we are able to collect only 30-40 modules it is always a starting point, release after release more libraries will be added, more people (who knows how to use a ngspice) would contribute and finally the eeschema-ngspice integration would be stupid (like me) free
This is how I imagine kicad 6 (or even 5.x)
edit 20/04/2019: i created a centralized repo https://github.com/kicad-spice-library/KiCad-Spice-Library and wrote a post on the forum Centralized repo for Spice libraries