I was thinking along the lines of Kerckhoffs' principle.
You seem to be conflating open source with key extraction or decryption ability. You seem to be saying that if a specification is known then that implies an ability to modify the system to a degree that allows those things.
You can have a system whose workings you know precisely, without obscurity, where the SW/HW specifications are open and the only secret is the key material. For instance, you could have keys be embedded in processor fuses. How would a user extract the keys in that scenario?
For DRM systems the user or consumer is the adversary, but the principles of secure system design still apply. You can still place an asset outside of adversary reach. For example, by ensuring that they need a super expensive FIB machine to read keys. Or, more traditionally, by tailoring cryptography to their computational power.
You seem to be conflating the malleability of software with its modifiability in a given system, but in reality those can be controlled. Yes, you can fork the software, but why are you assuming you can replace it on a given system?
Consider a black box with an LED. The box has an internal power supply and the LED emits light pulses in a given pattern pleasing to the user who purchased it. I could give you its physical properties and circuit diagrams. The box has data stored in internal memory. You can call parts of this data software if you like, as long as we agree that the output of the box is a function of this data and time.
The only contradiction inherent in DRM systems is that knowledge of the data/function needs to be given to the adversary and not given to the adversary at the same time. This has less to do with open or closed source, and more to do with whether the user can open the box.
I think that the "solution" to this contradiction has been reduction in fidelity, where the user/adversary is only given an approximation of the function (analog outputs that they have a hard time spreading to other would be users).
DRM sucks, but is not fundamentally at odds with open source.