Why? Because an economy with more and more automation based on private enterprise and private capitalist production could still sell its output and obtain money profits, if a government managed the demand-side of the economy by providing a guaranteed income (with, say, taxes on consumption, property, and ownership of financial and real assets and returns from those assets, with the shortfall covered by central bank money creation). As long as the balance of payments functioned successfully, a type of capitalism could continue.
That is, such an economy would still be a variety or type of capitalism: it would not be a command economy or the type of socialism envisaged by Marx.
Any Marxist response to this depends on Marx’s definition of capitalism. If one wants to define capitalism merely as a system of private production based on employment of free human wage labourers, then of course capitalism would cease once employment of free human labour ceased.
But this is just playing with words: setting up a narrow analytic definition of capitalism (true by definition), and ignoring other obvious real world aspects of capitalist systems of production.
If capitalism is to be defined in any empirically-defensible sense, it would need to use the following criteria:
Capitalism is a system of production as follows:Now if (3) fell and fell or even ceased to happen in an economy where production is increasingly done by machines, then it would still leave us with criteria (1) and (2).
(1) where the vast majority of all capital goods are owned privately and where there is a high degree of private property (in land, houses, private possessions, etc.) and rights to private property;
(2) where the vast majority of all decisions on investment and production of commodities are made by private agents (though this does not exclude certain public goods);
(3) where there exists a class of free human beings who work for a wage, either from the private or public sector (though mostly in the private sector).
The new system of nearly fully or fully automated production would in essence still be a type of capitalism, because it would still have traits (1) and (2), which clearly lie at the heart of what capitalism is.
In short, it would not be a system where all business is owned by the state or where the state plans all economic activity, and there is no necessary reason why a capitalist mode of production must end even if human wage labour falls towards zero.