Ever since neolithic times, when humans stopped gathering nuts and berries and started cultivating plants, the basic attitude to pests that afflict our plants has been the same, to bash, batter and poison them into submission. It is a method that never works in the long run. The reason is, in essence, just this. Complexity is necessary for stability in any association of plants, birds, animal and pests. When you reach for chemical solutions to the problem of pests, you simplify the system in your garden.
Simple systems are unstable.
Natural gardening is about accepting that with the plants you cultivate, you must expect to find a fairly complex insect life co-existing with it. It means that in some years, aphids for example, may be more of a nuisance than in others. In the long run however, natural predators of pests will control their spread. The aim of every gardener should be to control and not to eradicate pests within the garden, which is impossible anyway. The first principle of natural gardening is having healthy plants growing in a healthy soil.