Encoding is part of mastering of the disc, you have to compress those big movies to fit onto 50 GB with more extras than most people care about. You said Sirius Pixels was the best, I just provided 3 examples of where it failed with a very large bitrate size. I have a couple of custom Blu-ray discs mastered from DCP with x264 configured properly and the results are quite stunning compared to their retail releases. They also playback fine on a number of Blu-ray players. At the end of the day, most of those commercial encoders do not provide nearly the range of customization that x264 does, and they also aren’t updated as consistently being that they are closed source. By the way, it’s 2016. And the films I listed were on the front page of Sirius Pixels website as “excellent Blu-ray encodes” released in 2015.
I fail to see how poor authoring quality is the fault of the encoder. You could just as easily get a sub-par x264 encode by using x264 with insufficient bitrate (or a high CRF) as well. But I also gave you the example of MPEG2 encoders, where it is still clear that the best commercial quality encoders provide better quality at the DVD5 size compared to their open-source counterparts. MPEG2 has been around for 20 years now, so the free open source encoders have had plenty of time to get the quality to match or beat Mainconcept - but it just hasn’t happened, at least not yet. Perhaps one day. Or perhaps not, perhaps what we have now is “good enough” for the developers to stop trying.
Also, how many free MVC encoders do you see out there?
But in terms of x264 - quality is set by the CRF value. CRF-19 will always produce the same quality (roughly speaking). With slower values (the so-called right values) it will produce the same quality as the “fastest” preset but at a (slightly) lower bitrate. You can’t feed settings into x264 that will magically make CRF-19 look like CRF-16.