If you take all the factories in the world today, they could make all the parts necessary to build more factories like themselves. So, in a sense, we have a self-replicating industrial system today, but it would take a tremendous effort to copy what we already have.

The other advantage is that in conventional manufacturing processes, it takes a long time for a factory to produce an amount of product equal to its own weight. With molecular machines, the time required would be something more like a minute.

The basic parts, the start-up molecules, can be supplied in abundance and don't have to be made by some elaborate process. That immediately makes things simpler.

K. Eric Drexler Quotes