The old socialist doctrine, on the other hand, taught that all income from capital was "originally" due to labor and was "exploited" away from laborers by legal systems. And this is known to be false as a matter of economics, and unworkable as well. (Even a socialist economy must impute income to capital to allocate it efficiently, etc). Social democracy believed this socialist charge, and used it to justify expropriating from "the rich" by progressive taxation, as much as they believed the whole income of capital came to in their societies, then redistributing that amount as "social spending". This was buying the central economic fallacy of socialism and merely disagreeing about what means of addressing it was most efficient.
The same denial of the right to income from property, and the pretence that all of it "should" go to workers instead, runs through all modern radical ideologies. It exists on the modern populist right as the claim that income from finance is theft (through money doctrines), among anarchists and syndicalists, among Nazis, communists, socialists, social democrats, the modern liberal left with its goal of equality of outcomes. Many of these doctrines are more human or moderate than others, but at bottom all of them deny the legitimacy of income from property.
It is in fact the characteristic mark of conservatism that is recognizes and defends that right. This includes all the stripes of economic thought on the right with the exception of the finance-hating populist position mentioned above, and also of the moderate pro capitalist liberal left, say mid century Democrats, from Truman about to a Sam Nunn, say.