> The income of capitalists sure as heck isn't.

but where did the capitalists first got their capital? Why with labour of course! Their own labour, or their family's, or their grand-parent, etc.

I don't think that has any bearing on the response.

the capital that a capitalist spends is just unconsumed labour. It is directly related to the production of output (and thus income), just like labour is, because it _is_ another form of labour - let's call it solidified labour.

a system that doesn't directly link labour with income is basically describing welfare (a basic income for example). The income given to someone receiving this welfare did not produce any output that counterbalances the resources they received - aka, they are a net consumer. Without post-scarcity, there cannot be many net consumers, for what they consume, must be produced somehow by another (who is not a net consumer).

That is a much better argument for why profit is like welfare than how profit is like wages.