I have zero experience with requiring a stopping rifle but imagine that deep reliable penetration & the ability to place the shot under pressure makes the smaller cartridges at least equally effective as the perceived need for something beyond medium bore.
My Grizzly & Bear experience: I have flyfished Alaska 3x and have seen & been around grizzlies every time. Often we have to back off a riffle or break off a fish cuz they moved in. Hunted elk & mule deer in Wyomings grizzly country many times but have never run into one. Black bears are more common but they seem to be on the run away from me unless they are hitting my trash cans at home.
