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.
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.
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.
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.