> doubling the supply of cabbage on the market, which we would expect to more or less halve the price, leading to ... a zero sum game ;)

No. Noooo. This stuff is all connected. At a minimum, leading to consumers of cabbage spending less money in the market for foodstuffs, instead being free to spend the money elsewhere, leading to an increase in their personal well-being and economic growth in other sectors.

(And that's assuming that there is no change in the number of entrants into the home-cabbage-farming sector.)