Stephen Hawking has reminded us of why Marx is so relevant today. He's written:
If machines produce everything we need, the outcome will depend on how things are distributed. Everyone can enjoy a life of luxurious leisure if the machine-produced wealth is shared, or most people can end up miserably poor if the machine-owners successfully lobby against wealth redistribution. So far, the trend seems to be toward the second option, with technology driving ever-increasing inequality
There is, though, a problem with this second option: if most people are miserably poor, who will buy the products of super-machines?
This raises what Marx called one of the fundamental contradictions of capitalism. On the one hand, capitalism is great at developing productive potential. But on the other, demand might not keep up with this potential:
The conditions of direct exploitation, and those of realising it, are not identical. They diverge not only in place and time, but also logically. The first are only limited by the productive power of society, the latter by the proportional relation of the various branches of production and the consumer power of society. But this last-named is not determined either by the absolute productive power, or by the absolute consumer power, but by the consumer power based on antagonistic conditions of distribution, which reduce the consumption of the bulk of society to a minimum varying within more or less narrow limits.(Capital vol III ch 15)
This can lead to two possible outcomes. One is possible "realization crises" (see eg ch X of this pdf) if capitalists over-estimate demand and invest and produce too much:
The ultimate reason for all real crises always remains the poverty and restricted consumption of the masses as opposed to the drive of capitalist production to develop the productive forces as though only the absolute consuming power of society constituted their limit. (Capital vol III ch 30)
Another possibility, though, is that capitalists might fear a lack of demand and so not invest. (In fact, this is not the only reason why they might not invest. Investment is undertaken not by a representative agent but by individual capitalists. And each individual might be loath to invest or innovate for fear that future technical progress will undercut his own innovations.)
In either of these cases, capitalism will not develop technology by as much as is technically feasible. If so::
At a certain stage of development, the material productive forces of society come into conflict with the existing relations of production...From forms of development of the productive forces these relations turn into their fetters. Then begins an era of social revolution. (Preface to A Contribution to the Critique of Political Economy)
In these ways, capitalism is a form of collective action problem. We can imagine a society in which super-machines do indeed allow us all to live in luxurious leisure. But the decentralized decisions of capitalists might not get us there.
Granted, sensible aggregate demand policies might suffice to overcome realization crises - though the believe that such policies will be enacted is a form of what I've called centrist utopianism. But the other obstacle to investment and growth - the fear of future technical change - might not be so easily soluble within the confines of capitalism.
These issues are, of course, unresolved. What is clear, though, is that Marxism presents a useful perspective upon them.