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It would have been better if Marlin had simply added a little shoulder to the original .45-70 case. That way both the new cartridge and the original could have been fired in the more modern guns but the newer high pressure loadings would not have fit in older guns. In any case it would not have required a special case or draw. OTOH Marlin would not be able to corner the market on a new case design, I belikeve that was their real intent.
There isn’t enough taper in the .45-70 case to allow that – only 0.024” from the mouth to the rim. Such a design would essentially create a straight-wall case and feeding and extraction would be problematic. If you think such a small shoulder (0.012” maximum) would discourage everyone for chambering the new cartridge in a Marlin .45-70 you underestimate your fellow man.
The belt was actually a good solution. A longer case/COL wasn’t acceptable because of the action design and a fatter cartridge would have weakened the action by removing steel from the chamber walls. The belt prevents 450 cartridges from being chambered in a .45-70 chamber. While this may or may not have been an issue in Marlin actions, it most certainly was a concern with some other .45-70 actions.
Since Marlin does not manufacture either cartridge cases or ammunition, cornering the market was probably not their primary concern. That said, even if they had developed a case with a shoulder as you suggest they could have received a patent and demanded royalties. I suspect that as businessmen the primary concern of the management at Marlin was to sell more rifles while minimizing their risk. The belted 450 Marlin case did this nicely.