I suppose it comes from the fact that spores that create 'black spot' on the rosebush leaves, live in the soil at the base of the plant. If you don't remove all the black spotted leaves (especially fallen ones) before the rose bush goes dormant (or before spring), the spores will over-winter in the soil, and re-emerge when summer rain splashes them back up onto the leaves. This kills the leaves, and without enough leaves..no photosynthesis, and the plant dies.
A nice, deep mulch will keep the fungus from getting splashed back up on the leaves, or you can spray the leaves every 2 weeks in the summer with a fungicide.
I would think a good fungicide will kill enough spores, that planting a new plant in it's place, along with plenty of new soil should work fine. You need to inspect the roots of the dead rosebush for signs of mold (a white powder) when you dig it up. If that's the case, you may need to remove more soil, and dispose of it.