Two separate libraries that appear as one to the customer, allowing music to be played across libraries seamlessly
Which is exactly what the current implementation of iCML already does. It appears as one library to the consumer but internally music files are labelled whether they come from Apple Music or from other sources.
This way a customer can have one or both enabled. AM is the first library. iCML is the second library, and if iCML is disabled, the local music stored on the device is the second library.
I don't understand what you mean. There are two general sources for music: a) the one you are renting (ie, have added to 'My Music' from Apple Music) and b) the one you own (ie, the rest). iCLM is the umbrella that combines both together to make them 'appear as one to the customer'.
Could you try to explain again what system you are would like to see?
This way AM songs are never mixed in with music you own (so the system can never get confused and think your music is AM music), you can always view the libraries separately or together, and you can tell what's what in iTunes by the same column they're using now to tell you matched, AM, or uploaded.
So, the only thing you want is a 'column' in the Music app on iOS that shows whether the music is 'rented' or owned? Or do you want that on the Mac, 'rented' and 'purchased' music appear in separate folder structures? Which means you want to give the programmers at Apple advise as to how they should organise the internal structure of the data storage to make it easier to write a program without bugs?
Items downloaded to the device would still be identified the same way they are now (or more accurately, ideally), so there'd be no confusion about what's there or not there, regardless of which library it's attached to.
So you mean your suggested system works exactly as the current one, just minus the bugs?
This would allow customers to use all of AM without iCML or without wiping their personal libraries just to avoid uploading all their music (and having it FUBAR their personal libraries).
So, your suggestion here is that the current system wrecks havoc with the existing iTunes library by design?
It would also allow customers who want to sync their music the usual way to do so, and still be able to use all the parts of AM.
The moment the user has to look at every song to see whether it will be automatically available on iOS devices or whether he or she has to make a decision on whether to sync it, I don't think we can talk anymore about a library that appears as one
I am fully with you that having an ability to set in iTunes which songs are stored locally on a (synced) iOS device would make things much easier (even if that is completely optional, ie, you can still do exactly the same on the device itself if you so prefer or don't even have a computer).