Although IMO opinion John Hicks, a good photographer and a great person I bet, talks about a couple of relevant, valid things in his "comments", some of his words there don't seem to reflect experience in real low light photography... I repeat: in my opinion.
Examples: medium format is never the best option for low light + moving subjects. If you do landscape of any kind and you use tripod, no problem of course, but, f/2.8 for low light? I hope he's not talking about handheld photography. When light is low, even at f/1.4 @3200, speeds are low, like 1/30th and 1/15th. In those cases f/2.8 is totally useless if we need people without blur.
35mm and any f/1.4 lens are much better tools. Plus, we get a lot less DOF, at the same f-stop, if we use medium format instead of 35mm.
And what he says about rectangles made me smile: we don't use 6x6 to crop for a rectangle, but to compose for a square. We compose for a rectangle with 35mm or with 6x9.
6x7 doesn't replace 6x6, and it doesn't replace 6x9 either.