I think that another thing that we forget is the fact that many people didn't have even one firearm, let alone the luxury of having two that would need the same ammo.
Many a homestead only had a shotgun, if that, for a weapon, and the cowpoke who could afford even one firearm, rifle or handgun, was lucky indeed. Many still carried old C&B revolvers long into the cortridge era.
As a result, interchnageability of ammunition wasn't a big deal. Throw in the proprietary issues and "turf fight" between Colt/Winchester/S&W/everybody else, and it didn't make economic sense to chamber rifles in .45 Colt.
Even though we often claim (and rightly so) about the infringements on our RKBA, we are lucky indeed to have both the choices and the economic power to own the firearms we do. For example, I have enough of the right firearms to play in SASS, but if I wanted to play "Ninja Tactical SWAT" I could do that, or WWI GI, or WWII Tommie, etc. Additionally, I could do all of that and never have to use my hunting and plinking guns. Amazing that the "Wild West" had fewer firearms per capita than we see today.