Is there any mechanism for it to validate that the thing connected to the display output is an unmodified monitor which also implements DRM? Otherwise it would be almost trivial to have a device with HDMI-in that gets a pixel perfect feed of the protected stream.

Yes, that's called HDCP. The master key was cracked long ago for older versions.

For new versions, you can get a converter; hdcp 2.2 -> hdcp 1.4 and then use whatevee breakage on 1.4. So it's only effective if you don't search.

You can’t produce good quality rips like this, if that was the best solution the DRM people would have won.

You can produce pixel perfect rips. But since the resulting video file is so large you will "need" to compress it, and then you get compression on compression artefacts.

Yes, indeed. Pixel perfect rips of already compressed streams, immensely blowing up the size and requiring another round of lossy compression to actually be useful.

And the result after recompression is impeccable so it is not a big deal. It is just a bit cumbersome.

The result after recompression is far from impeccable.

On high bitrate content you wouldn't notice.

Sure, but you get to push your encoder settings far less than you otherwise could. You always end up with either significantly bigger files or significantly lower quality.

It's significant closer to 5 percent or 50 percent?

It would be really interesting to make a video compressor that specializes in "recompressing" previously compressed data. A simple version could be special handling of common compression artifacts and trying to detect various transforms and trying to take advantage of the detail already removed instead of removing additional detail.

If you knew the codec used originally you could even try to (near) perfectly reconstruct the original bitstream. Especially if you know various codec parameters and some side-channel data (such as which video data is contained in which encrypted blocks) it may be possible to relatively efficiently search for the bitstream that decompresses to the data that you are seeing.

doubt it, should be on OS level, gpu control for all possible vendors and output formats is impossible without gpu driver support.

It's specific to the Intel GPU. Intel PAVP (Protected Audio-Video Path). See page 192 of Intel's "Platform Embedded Security Technology Revealed" (2014), https://link.springer.com/book/10.1007/978-1-4302-6572-6