The Fifth Amendment states "[No person shall] be deprived of... property, without due process of law; nor shall private property be taken for public use, without just compensation."
In other words, this means a lawsuit for every existing software patent to determine its hypothetical value were the law left as is. The Federal Government is suddenly the defendant in millions of civil cases. It will essentially have to buy every software patent out there. The fallout would destroy everything you love and more.
I know it seems like a sad compromise, but there might be huge practical difficulties with that sort of ex post facto rejection of existing patents. Just tell them to stop issuing new software patents, and urge the judiciary to vigorously scrutinize existing software patents which come before the bench for obviousness or prior art.