"Marx never asked what might happen if intense global competition some time in the future forced entrepreneurs to introduce ever more efficient technologies, accelerating productivity to the point where the marginal cost of production approached zero, making goods and services "priceless" and potentially free, putting an end to profit and rendering the market exchange economy obsolete."

Uh, Marx did ask that question. It's called the tendency of the rate of profit to fall.

The problem with increased productivity and lower marginal costs is that labor, too, becomes less and less valuable. Supply of labor goes up; demand of labor goes down. People are more easily exploited.