If it's open source, I can just swoop in, inject some code and make a copy of the decrypted data stream for 'archival purposes'. Won't take a day for a PoC.
I can still do this with closed source DRM blobs, but it will take much longer. And there will probably be pointless anti-debugger tricks, system wide hooks that break countless other software, kernel drivers that BSoD your system..
That is precisely why this proposal is such a terrible idea. It writes into a standard that it is okay to produce software that is actively hostile to its user, while having absolutely no security gain whatsoever (because the concept is fundamentally broken: if the data is being decrypted on my system, I will get it).
