Re: Oh dear, a system to change the *whole* business will be done as just a S/W change out.
I went anonymous on this as I worked on these systems back in the '70s & '80s. When the system was designed the capabilities of other databases on the 370 based platforms had some limits that could not be matched to the requirements, ADABAS was also another contender but Model204 had greater capabilities and were demonstrably able to meet some of the other requirements. Model204 had a number of other large govt departments in the US and UK who provided some feedback which may have influenced the decision also but I can only say that from an interested observer perspective.
With regard to the scheme being documented, last time I looked the schema specification had it's own life support system, was documented, catalogued, etc. Of course I cannot comment on the last 30 years or so.