Team,
I think there are two points here:
1) the current DB structure (considered as a whole) have issues and the library management (that is super core to Kodi) does not work well, does create duplicates, does not allow to be centralized on a nas, etc...
2) which is the best way to implement a new schema?
Now for point 1) the new schema does provide quicker queries, better data, lesser duplication, etc. It creates the correct separation between the data groups while sharing core and common data
Regarding 2 on separate or joint databases that's a different story. The idea that having a single db will create lock downs is not completely correct as anyone can develop their own part without messing up the others.
For example we added all the games part after the initial design was done with no impact.
The fact is that there are common parts that cannot be separated and must be kept together.
For example the users and their visibility of objects is something cross the DB (or all the DBs).
The files are cross all the DBs
If you separate them you'll end up (again) with a mess where files are handled differently between games, audio, video, series...
You'll not be able to manage correctly the visibility of objects between the database or to do it you'll need a massive amount of code while on the DB is just a where clause
I've not studied all the KODI code, as is massive, but the first analysis I did before starding to redesign the DB had identified that there were very specific points where the queries are executed.
What will chage are only those points as the new structure is able to return (at a first implementation) exactly the same data.
After that it will be possible to enchance and take advantages of the new structure
Just my 2c but as Kodi is advancing very well the drawbacks on the library management, lack of a centralized db etc.. are starting to became a real haeavy issue...
Also note that as the light witght devices (raspberry, low power HTPC, etc..) are continuously growing Kodi really needs to move out of the core all computational workload. All NAS I know do have an on board DB that can offload kodi from all of that and become much more responsive.