One doesn’t even know where to start, other than criticize a completely unsupported proposition.
First, if we take “socialism” to mean the protection of the poor without respect to their competence at competing in an unregulated society, then it’s fair to say that socialism in some modern countries has succeeded quite well. Socialist policies in many European nations have produced a very good support system. One notes that its success in countries considered highly socialized such as Sweden, is more successful when the population is homegeneous. In recent years immigrant populations which are less competitive (academically, especially) still struggle to be other than heavily supported “poor.”
If we consider the notion that a country can–as a unit of commerce–be capitalist, then capitalism ALWAYS wins. Once you broaden out to include the world as the “poor” that need supporting, then only a capitalist nation will thrive on the world stage.
So the “socialist” notion that socialism is somehow superior because it supports the poor best (and I’ll leave aside the weakness of the statement that that is how one measures prosperity) extends only as far as a nation’s border.
Were Sweden–or any other nation–to decide that all humans worldwide need supporting to the extent that Sweden supports its own, then socialism collapses the nation which practces it immediately while Capitalist nations thrive.
At some point, for economic success, a nation needs to function as a single narcissistic unit above the altruistic idealism that the global poor should get supported beyond the way we support any beggar.