I have a 45-90 sitting on the bench, place a 45-70 next to it and the bullet of the 90 basically sits at the top of the mouth of the 70, so the 90 is a 'full case o' powder' type of thing.
If you work out the proportions of dia/case capacity/bullet weight, etc using that as a guide you can maybe calc that out and create the '380 Uper'?
Mostly though, I think that the advent of the more powerful smokeless powder and the move to 'small bore' calibers did it in. The old timers held onto theirs but the young turks took up the 30cal.'s and if they needed more beans, maybe went straight to the 45-70. The schuetzen target shooters moved from the 45-70 to 38-55 to 32-40, etc. because they could shoot good scores longer with smaller calibers.