Completely agreed
@WAB. Same goes for cape buffalo etc. This discussion goes back all the way to 'Pondoro' Taylor's 1948, what did it call it? "KO factor" if memory serves, and my personal field experience, although infinitely much more modest than Taylor's LOL, absolutely supports his perspective, 'Doctori' Robertson's, yours, and many others' who have
actually hunted dangerous game.
The challenge in this thread is "One rifle for North American big game," therefore, by definition, the answer will be a compromise, which, again by definition, is not ideal in ALL situations. Of course, I would take a .375 (H&H, Wby, RUM, Ruger, etc.) any day over a .300 (H&H, Win, Wby, RUM, WSM, etc.) for dangerous game, and truth be told - believe the timing of this discussion or not ;-) - I am flying tonight to Africa with a cape buff on license, and my caliber of choice is not even the .375 H&H but the .470 NE. Now, how is THAT, for TOTALLY agreeing with you that "they are much more impressed with size than speed" LOL?
But I think that neither you or I would recommend the .375 as "One rifle for North American big game" (although truth be told, if one was willing to limit oneself to 200 yd, the good old .375 H&H would do quite well on pronghorn, as it has done for over a century on everything that walks in Africa, as THE "one rifle safari" gun since 1912...). And yes, we both know that a great many elephants were killed by 'Karamoja' Bell and others with the 6.5x54 Mannlicher Schoenauer or 7x57 Mauser, but we still won't recommend it for dangerous game, correct? So, for lack of a .375 with 300 gr premium bullets as "One rifle for North American big game" I will still, and I suspect you will too, favor a .300 with 200 gr premium bullets, or a .338 with 250 gr premium bullets, over a 7x57 for brown bear, right?