What about the contemporary "theory" of the "80-20 society", according to which suipposedly 80% of all _paid _labour_ power_ (let alone domestic labour etc) at present is what the classical economists called "unproductive labour"? (Marx pointed out in _Grundrisse_ that Adam Smith was consistent: he included the clergy in UPL...) Perhaps such ideas mainly serve to soften us up for further deregulation of labour markets ... But (for example) a friend who works in workers' compensation personally witnesses hundreds of working hours squandered every month, in court cases between several insurance companies, all arguing over which one should pay up. There are plenty of other examples.
So doesn't competition duplicate and fragment socially necessary work already? Wouldn't it mean that socially necessary work would be extremely minimal, in any postcapitalist society, as long as it succeeded a _fully_developed_ capitalist economy?