Did you ever see clips or read about Eddie Shore's ability to skate backwards? No and that explains the lack of mobility of defencemen.
Red Line, introduced in 1943,created two zone forward passing. End boards to the Red Line and between the Blue Lines so suddenly defencemen had to improve their skating. Dynasty Leafs were the first to change bringing in rookie d-men for 1946-47.
There were parallel tracks, slightly different philosophies, approaches to the game between Upper Canada & the West, Lower Canada & Quebec in terms of player development, absolutely. The former you had some early Rushing Defencemen, often times converted Forwards to Defence, whereas in the case of the latter Defenders much more stationary, the speed & artistry all up-front though in-built, as was the case in Upper Canada & the West, forwards who were defensively responsible. During this period, a real hot house effect in Quebec amongst the goaltending fraternity, innovation of "wandering" (Plante et al) in "assisting" their less than mobile Defencemen. Playing the puck. Borrowed, bit of a throwback to the age of Joe Hall & the old Quebec Bulldogs but with some twists of course.
Plante a "Wanderer" as a result of playing with Defenceman who not only couldnt skate backwards, they couldnt turn to their off naturally strong skating side without collapsing, face plant, yard sale, BREAKAWAY, He Shoots, He Scores. From there it just grew, morphed.... "Fast Eddie" Giacomin, an outlier if ever there was one (overlooked as an amateur, stove blows up in his face causing 2nd & 3rd degree burns etc etc etc) , from Industrial Hockey in Sudbury to a walk-on in the EHL & the NHL, another guy who could skate, Roamer. Usurped, displaced one time Leafs & Hab prospect Cesare Maniago in NY (Emile Francis realizing this weakness & bringing in Giacomin with whom he was mightily impressed, Francis knowing a thing or 3 about the game, Goaltending). Adaptation, being aggressive, name of the game. From the crease out. This was critical, crucial to the development of the game....
Wild Bill Durnan. Dr. Strangeglove. He too a Happy Wanderer on occasion, tactically, working with, in total unison with his lugubrious Defenders. The "Flying" in the "Flying Frenchmen" all up-front, in your face. Stop things before they get started with a mobile Goalie whereas in Upper Canada & Western Canada at the Minor Pro & Junior/Senior levels, mobility on Defense encouraged more dependent however on the organization & its braintrust. Regardless, there is a reason why the Quebec School of Goaltending is so much more venerated and this aspect but one of several responsible for it being so... and if you drill down even further even within Quebec and its various regions there you will find other schools of thought, styles of play. Some with an emphasis on skating skills on the backend, less reliance on Goaltender mobility, integration of Offensive Defencemen with their Offensive counterparts on the Wings, Center. Always made for interesting opponents if like me you were from Southern Ontario.... and of course if we couldnt beat you boys through adaptation & skill... more than happy to do so with our sticks, fists..... Uh? ... Ya, you heard me Big Boy. Bring it.