There should be no need for 6.1 as the original source format of the matrixed 4.0 perfectly fits into the 5.1 system by having the rear surrounds recreate that back center channel as the rear is simply phase split to repurpose the original track into a stereo surround setup without changing any content. This is how it is supposed to be done in the professional realm.
Also the discrete 70mm source would work in a similar way by having the front LCR and the rear mono surround merely split across L and R surrounds.
What is bad about 6.1 is that the center rear naturally creates listener confusion as it makes the sound from the center mix with the rear center and then the sound doesn’t come across as intended. This is why they dumped 6.1 in favor of 7.1 many years ago and the fact that the audio mixes are still stuck in this outdated format on the BDs is a further sign of the lack of care and incompetence shown throughout their transfer history since 2004.