You hit the nail on the head as far as I'm concerned. I would have to think that the PH would want, first and foremost, a client to have a gun/cartridge that the client could accurately shoot! Not only for the first critical shot, but for any fast/furious lead-slinging that may happen if things don't go as planned. After all, in a charge the hunter may have the opportunity to help settle things, as well as the PH.
The PH should be selecting their arms on the same basis, but logically, we might assume they can handle more firepower. If so, great. But for the average hunter, things are going to go a LOT more smoothly if they show up with a .338, or even .30-06, that they can handle, vs. a .458 that they are afraid of. Heck, my .35 Rem through a leopard's shoulders beats the heck out of missing with ANYTHING!
A .45-70 through the shoulder/lung area is much more sporting than a .458 Win Mag through the guts... and frankly, anyone who doubts the effectiveness of hard cast bullets probably hasn't tried them! I have gotten several FEET of penetration.... from a HANDGUN.... now what sort of increase do I get when I add 100 grains of bullet weight and 800fps from a rifle?????
The only game I think that I might be a little hesitant to shoot with a heavy cast bullet is an elephant, as I don't know that such a bullet would go through several feet of bone in an elephant's skull (it might but I don't know). In that case I'd just dispense with conventional lead core bullets as well, and go for a 'monolithic' solid like the Barnes or A-Square.
As Scotty points out, a reluctance on changing what has been proven to work may be why the PH's are staying with .458/500gr. solids. Frankly, if I had been a PH for years and had learned to use that combination, I'd be a little reluctant to experiment (bad environment to risk a mistake!). I have also read that some PH's have gone to the Marlins for thin stuff like lions (which really aren't all that big in the scheme of things) but I can't remember where I read it.