I don't think building a business is zero-sum in principle. Your point about a farmer creating value is well taken. Our present measures of economic value are notoriously short-sighted though (i.e., willing to ignore hard-to-quantify externalities); did the green revolution create value? It vastly improved yield, but if we don't make serious changes in the next 50 years, it will destroy the viability of agriculture. Value creation is rarely unambiguous.

I would also argue that many VC-backed startups are more about finding unexploited niches or catabolizing existing sectors of the economy than creating new material wealth.