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The socialist slogan is - as Marx pointedly remarked in "Critique of the Gotha Program" -, not an argument for equality but "from each according to ability, to each according to their need"; for substantial parts of the left, the idea of society paying people if they're not contributing is directly in opposition to a central concern of their ideologies to prevent people from doing just that.
As careful as one should be about talking for the dead: Had Marx been alive today, he'd probably join you in opposing UBI, for similar reasons.
Rather, the idea stems from people like Thomas Paine, and was bolstered by experiments in the US and Canada in the 60's.
Recently it was rediscovered first and foremost by liberals (in the European sense; in other words mostly centrists, again by European standard, followers of classical liberalism) explicitly as an alternative to left-wing welfare systems.
> "opposition to enabling people to live off the back of others"? strange, the left implements that everywhere already.
"The left" is not a single thing, but represented by dozens of ideologies that are wildly different.
No shit. He won't. Not unless you pay him 300,000/year and that's a whole other kind of economic catastrophe.