Slavery is the basis of capitalism, where you need start-up capital, the easiest way is to get cheap labour, here is the surplus you need to get the machine going.
But if you stay in a closed system at some point the purchasing power equals the wealth produced, the money in circulation, the wealth is always the same, you can't make more than what you put into circulation.
Hence globalisation, new markets from which wealth flows, in the long run the same reasoning applies, wealth is finished.
Capitalism is doomed to fail, in fact it has already failed, we will suffer its tail blows for a few more decades, at the very least.