My standard bear gun is a Marlin 1895STP in .45/70 loaded with 405gr hardcast lead pills. However, I've carried it mostly for defense purposes though I also carried it for "backup" purposes for my buddy who took a Kodiak Brownie with a bow without incident.
IMO, the last thing in the world that you want when hunting a big brown bear is a whizzum-bang-earschplittenloudenboomermagnum. I have a personal distaste/disdain for the .300 Win Mag, it won't do anything a .30/06 or .308 won't do in a realistic hunting situation with proper ammo. Just because the same weight bullet is going faster doesn't mean it'll kill a bear quicker, and the increased speed is actually counter-productive with all but the toughest bullet designs.
I don't completely understand your logic. The one point where we agree is that a 300 mag doesn't offer any advantages at close range over a properly loaded 308 or 30-06. The extra velocity just adds effective range, not close range killing power.
But I think your 45-70 theory is flawed. While it does not have "magnum" on the headstamp, when you get to the power levels needed for large bear, it is a magnum.
A 220 gr Nosler partition fired at 30-06 or 300 mag velocities will have no trouble holding together and has proven to out penetrate 45-70 loads.
Then there is the recoil factor:
My 8 lb 30-06 loaded with 220 gr partitions @ 2500 fps will generate 24 ft. lbs. of recoil.
My 8 lb 300 WSM with the same bullet @ 2650 will generate about 28 ft. lbs. of recoil.
My 7.5 lb 45-70 loaded with 405 gr bullets @1850 fps will generate over 40 ft. lbs. of recoil.
I can't see having a round that has close to double the recoil that does not offer me any performance advantages over a 30-06. While you have a disdain for a magnum, it actually does the same job with far less recoil. Sure, if you are using a 300 mag with light 150 gr bullets @ 3400 fps they will not get the job done, but neither will a 300 gr 45-70 bullet @ 2500 fps.
My personal choice would be the 30-06 because of easy availibility of good ammo and the fact that at closer ranges the 300 mag would offer no real advantage. But if I happened to have the 300 in my hands it would not be a disadvantage and would be a slight advantage if a longer shot were needed.
I've owned a Marlin 45-70 for close to 40 years and it would stay at home. I have no doubts that a properly loaded round would get the job done, but it wouldn't do it a bit better than many others, with much less fuss and recoil.
The biggest problem with the 45-70 is not the chambering, but the package. At those recoil levels I can shoot a bolt rifle faster for repeat shots than a lever action. I've simply had too many reliability issues with lever guns to trust my life to one either. When you get into really hot loads in any action type you are asking for trouble but bolt guns handle them much better. They are also by far the most bulletproof and reliable, especially under harsh use and weather conditions encountered on such a hunt.