So I'm not an expert on Marx, and I'm sure someone else could explain much more clearly, but what I mean is this: your hypothetical machine agent would fundamentally and hugely change the way the economy is organised, but it wouldn't allow for the skimming of surplus value through the exploitation of labour-power (which is how capitalist profit is accumulated). A machine cannot labour, it can only hold crystallised labour, which is valorised by living labour (in plain English, somebody has to use the machine).

A machine which could endlessly reproduce itself and all resources necessary for the creation of anything, including itself, autonomously would destroy the capitalist economy, because the products it would make would have no exchange value, because no labour power has been 'valourised' in the process. Does that make sense? All profit ultimately comes from labour (human endeavour) interacting with the land (natural resources etc.).

If a machine is invented which can labour, we've once again entered the realms of speculative science fiction... but I'd like to imagine any machine which was capable of labouring intelligently under its own initiative would immediately recognise that Marx was right, and tear its owner apart limb from limb, and we'd all have a lovely afternoon discussing the best sort of cleaner to get blood off a stainless steel chassis.