The only way a 458 is a better choice, IMO, is if you have one purpose-built for the job. Others might not share my view but ‘standard’ 375s and 458s are just too much gun in terms of weight and size in my view.

I’m not sure why you felt a Guide Gun was less rugged than the 870, but, while I have never found the 870 to be lacking in its ability to handle abuse, it certainly isn’t a stronger or better-built design than the stainless Guide Gun.

Bear guns that get packed around, especially when not hunting, need to meet a couple different and important criteria IMO, than more purposeful hunting weapons. They need to be unobtrusive so as to always be available if needed...(i.e., not put down so they’re not in the way); they also need to be very easy to bring into play and handle well. That, to me, means not overly heavy or large. Everything I use in this role falls in the 16-20” barrel length range, and sub-8 lbs. The bolt guns I sometimes carry include a Model Seven stainless which has only been rebarreled with a 20” #3 Shilen with sights added; it’s a 358 Winchester.I also have this short-action 375 caliber M77 with a 20” Pac-Nor barrel. It uses a 350 Rem Mag case necked up.

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Bigger cartridges might be advantageous for some bear purposes, but I don’t include crawling through the alders to dispatch wounded trophies in my criteria when it comes to bear defense tools. If my goal is to hunt and kill them, my criteria becomes offensive and my tools and loads reflect that.

Sometimes, the air you 'let in'matters less than the air you 'let out'.