I think that you could call it "traditional" but if so I would call your other two examples, the 465 H&H and 318 WR "classic" Cartridges. They are all African cartridges for sure but from different eras. Would the 458 WM ever have become as popular in Africa without Knock getting out of making ammo for the "classic" heavies? Maybe, maybe not. But the 458 in affordable bolt actions opened up African hunting with heavies to many who would otherwise been unable to pay for the pleasure.

I read alot about the 458 being "marginal", but wouldn't one make a great first time big bore to own? Cheap Ammo(relatively) and able to fit shorter actions? Handloading componenets are easy to obtain. Wide bullet selection.

I have a Ruger MArk II in 7mm Remington Mag that I just do not use that much anymore. It shoots great but I just have lots of other rifles and actually do most of my hunting with a bow. I was thinking about turning it into a big bore for a future African buff hunt (still a dream

) Because it will only handle up to a 30-06 length action teh 458 WM is a natural.