Also, the amount of tools every single person would need to buy.
"Decentralisation" is a goof idea, but not feasible in the real world, IMHO. And this is a small town example, leave alone a huge capital like Madrid or Barcelona.
In Britain and Germany, allotment gardening was promoted as a social measure and by all accounts it was successful.
In 2019 I produced almost 300 Kg of my own food at home without really trying hard, so maybe not all year round, but is possible to notice a difference. The garden had provided me with at least a piece of fresh fruit each day of 2020 and this point is covered until june-2020 at least.
The problem is trying to obtain your food without space, without feeding your plants, and relying in magical products. This never works as intended.
People needs to understand also that producing food at home is more expensive (but the quality is better).
Fragile systems break more easily. Make sturdy systems humans rely on, even if more costly and less efficient.
It's also amazing how little land is needed to grow enough food to live on. One person can be fed from around 9 square meters. In dense cities people may not have that much land, but in many areas, people do have that much land in their yards.
My wife's parents grow various vegetables in an area of that size, and there is no way it could provide all their vegetable needs.