... for whatever reason the MF world decided that the only way they could compete with DSLR is with 645
It has little or nothing to do with digital: the MF world was moving away from square formats to oblong(*) formats like 645 and 6x7 well before the digital transition: Rollei and Hasselblad were losing market share to Japanese upstarts like Mamiya, Pentax and Bronica, all of which offered mostly or exclusively non-square formats. (The one exception: Bronica SQ, but the 645 format ETR series was Bronica's best seller.) And Hasselblad started its transition to 645 and AF back in the film era.
Digital merely adds a nail to the coffin, since sensors have to be made in a particular shape, whereas the same roll film can be used in a variety of format shapes. So it just becomes more manifest how few images taken on 120 film were being composed for or printed in square format.
Digital has been responsible for leaving behind 6x7 (and 6x8, 6x9, 6x17 ...), due to the vastly greater cost of such large sensors in exchange for IQ improvements that would only be noticeable in too small a portion of photography to make it economically viable. (RED claims otherwise, but let us see what RED actually achieves in any format larger than its current 30x15mm maximum.)
* Nitpick: a square is a kind of rectangle: if you mean "a rectangle other than a square", the best word is "oblong".