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).