I think the standard logic would be along the lines of what Zundfolge said knowing you will actually be in an immediate self defense encounter.
But...
Note: I'm not talking about hunting grizz, but defense from the grizz.
This changes everything.
The gun you have with you is the one that will protect you on that rare occasion.
The .454 comes in a much more compact package, allows familiarization with less expensive and widely available .45 colt, and is far more likely to actually be carried on your hip if you end up needing it. It can also be handloaded anywhere in between, using both commercial .454 bullets or those designed for .45 colt velocities, and so may be tailored to specific uses or regional threats.
It can also double for self defense against humans with .45 colt, filling a role in more environments.
It is has performance not far behind the .500S&W in .454.
After people hike around in the rugged outdoors with a massive .500 many times without ever needing it but quite aware of its uncomfortable bulk they typically start to leave it behind.
The gun left behind won't do you any good, and so the gun that will actually get carried is a better defensive gun.
Being agile and climbing, hiking, backpacking, or exploring in the outdoors with a giant .500 on your hip is just not going to be enjoyable.
Once the uncomfortable monster gets left behind or stowed away in your pack as a result it ceases to be available for immediate unexpected self defense.
The same can be said for long guns, a practical sidearm you will actually carry is far more likely to save your life because the long gun will often become a burden and get left behind. Or set down while skinning/cleaning/filleting or hauling something.
So the best self defense rifle leaned up against a tree several feet away while you deal with real life chores requiring two hands might as well be miles away when the bear comes calling unexpectedly.