Even in preschoolers whose habits are being established, existing dietary attitudes are relatively resistant to change. how should wise parents react?

I believe the answer is: Be patient and persistent

In such situation, parents should realize that since habits are developed overtime, they also need some time to revert/change those habits.
One thing that the parents do is start to make small changes in the dietary food (such as adding a little vegetables) and started to increase the quantity over time.